Move type:module declaration into package.json. (#4355)

* Rename `switch_package_to_release.js` to `.cjs`

Slightly surprisingly, the symlink is enough to make `node
switch_package_to_release.js` work.

* Rename .eslintrc.js to .cjs

Again, declare this as commonjs

* Move `type:module` declaration into package.json.

matrix-js-sdk is built into ECMAScript modules, and we should declare it as
such. See https://nodejs.org/api/packages.html#type. Failure to do so causes
problems for javascript projects attempting to build against matrix-js-sdk: see https://github.com/matrix-org/matrix-js-sdk/issues/4347.

Previously, we did this as part of the package.json switcheroo, but that is
unnecessarily fragile.

matrix-react-sdk, element-web, etc are unaffected by this, because they use the
typescript files directly, by importing `matrix-js-sdk/src/...`.

.eslintrc.cjs

@@ -0,0 +1,142 @@
+module.exports = {
+    plugins: ["matrix-org", "import", "jsdoc"],
+    extends: ["plugin:matrix-org/babel", "plugin:matrix-org/jest", "plugin:import/typescript"],
+    parserOptions: {
+        project: ["./tsconfig.json"],
+    },
+    env: {
+        browser: true,
+        node: true,
+    },
+    settings: {
+        "import/resolver": {
+            typescript: true,
+            node: true,
+        },
+    },
+    // NOTE: These rules are frozen and new rules should not be added here.
+    // New changes belong in https://github.com/matrix-org/eslint-plugin-matrix-org/
+    rules: {
+        "no-var": ["error"],
+        "prefer-rest-params": ["error"],
+        "prefer-spread": ["error"],
+        "one-var": ["error"],
+        "padded-blocks": ["error"],
+        "no-extend-native": ["error"],
+        "camelcase": ["error"],
+        "no-multi-spaces": ["error", { ignoreEOLComments: true }],
+        "space-before-function-paren": [
+            "error",
+            {
+                anonymous: "never",
+                named: "never",
+                asyncArrow: "always",
+            },
+        ],
+        "arrow-parens": "off",
+        "prefer-promise-reject-errors": "off",
+        "no-constant-condition": "off",
+        "no-async-promise-executor": "off",
+        // We use a `logger` intermediary module
+        "no-console": "error",
+
+        // restrict EventEmitters to force callers to use TypedEventEmitter
+        "no-restricted-imports": [
+            "error",
+            {
+                name: "events",
+                message: "Please use TypedEventEmitter instead",
+            },
+        ],
+
+        "no-restricted-properties": [
+            "error",
+            {
+                object: "window",
+                property: "setImmediate",
+                message: "Use setTimeout instead.",
+            },
+        ],
+        "no-restricted-globals": [
+            "error",
+            {
+                name: "setImmediate",
+                message: "Use setTimeout instead.",
+            },
+        ],
+
+        "import/no-restricted-paths": [
+            "error",
+            {
+                zones: [
+                    {
+                        target: "./src/",
+                        from: "./src/index.ts",
+                        message:
+                            "The package index is dynamic between src and lib depending on " +
+                            "whether release or development, target the specific module or matrix.ts instead",
+                    },
+                ],
+            },
+        ],
+        // Disabled tests are a reality for now but as soon as all of the xits are
+        // eliminated, we should enforce this.
+        "jest/no-disabled-tests": "off",
+        // Also treat "oldBackendOnly" as a test function.
+        // Used in some crypto tests.
+        "jest/no-standalone-expect": [
+            "error",
+            {
+                additionalTestBlockFunctions: ["beforeAll", "beforeEach", "oldBackendOnly", "newBackendOnly"],
+            },
+        ],
+    },
+    overrides: [
+        {
+            files: ["**/*.ts"],
+            plugins: ["eslint-plugin-tsdoc"],
+            extends: ["plugin:matrix-org/typescript"],
+            rules: {
+                // TypeScript has its own version of this
+                "@babel/no-invalid-this": "off",
+
+                // We're okay being explicit at the moment
+                "@typescript-eslint/no-empty-interface": "off",
+                // We disable this while we're transitioning
+                "@typescript-eslint/no-explicit-any": "off",
+                // We'd rather not do this but we do
+                "@typescript-eslint/ban-ts-comment": "off",
+                // We're okay with assertion errors when we ask for them
+                "@typescript-eslint/no-non-null-assertion": "off",
+
+                // The non-TypeScript rule produces false positives
+                "func-call-spacing": "off",
+                "@typescript-eslint/func-call-spacing": ["error"],
+
+                "quotes": "off",
+                // We use a `logger` intermediary module
+                "no-console": "error",
+            },
+        },
+        {
+            files: ["src/**/*.ts"],
+            rules: {
+                "jsdoc/no-types": "error",
+                "jsdoc/empty-tags": "error",
+                "jsdoc/check-property-names": "error",
+                "jsdoc/check-values": "error",
+                // These need a bit more work before we can enable
+                // "jsdoc/check-param-names": "error",
+                // "jsdoc/check-indentation": "error",
+            },
+        },
+        {
+            files: ["spec/**/*.ts"],
+            rules: {
+                // We don't need super strict typing in test utilities
+                "@typescript-eslint/explicit-function-return-type": "off",
+                "@typescript-eslint/explicit-member-accessibility": "off",
+            },
+        },
+    ],
+};