Use & enforce snake_case naming convention on config.json settings (#8062)

* Document and support the established naming convention for config opts

This change:
* Rename `ConfigOptions` to `IConfigOptions` to match code convention/style, plus move it to a dedicated file
* Update comments and surrounding documentation
* Define every single documented option (from element-web's config.md)
* Enable a linter to enforce the convention
* Invent a translation layer for a different change to use
* No attempt to fix build errors from doing this (at this stage)

* Add demo of lint rule in action

* Fix all obvious instances of SdkConfig case conflicts

* Fix tests to use SdkConfig directly

* Add docs to make unset() calling safer

* Appease the linter

* Update documentation to match snake_case_config

* Fix more instances of square brackets off SdkConfig

test/utils/SnakedObject-test.ts

@@ -0,0 +1,63 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { SnakedObject, snakeToCamel } from "../../src/utils/SnakedObject";
+
+describe('snakeToCamel', () => {
+    it('should convert snake_case to camelCase in simple scenarios', () => {
+        expect(snakeToCamel("snake_case")).toBe("snakeCase");
+        expect(snakeToCamel("snake_case_but_longer")).toBe("snakeCaseButLonger");
+        expect(snakeToCamel("numbered_123")).toBe("numbered123"); // not a thing we would see normally
+    });
+
+    // Not really something we expect to see, but it's defined behaviour of the function
+    it('should not camelCase a trailing or leading underscore', () => {
+        expect(snakeToCamel("_snake")).toBe("_snake");
+        expect(snakeToCamel("snake_")).toBe("snake_");
+        expect(snakeToCamel("_snake_case")).toBe("_snakeCase");
+        expect(snakeToCamel("snake_case_")).toBe("snakeCase_");
+    });
+
+    // Another thing we don't really expect to see, but is "defined behaviour"
+    it('should be predictable with double underscores', () => {
+        expect(snakeToCamel("__snake__")).toBe("_Snake_");
+        expect(snakeToCamel("snake__case")).toBe("snake_case");
+    });
+});
+
+describe('SnakedObject', () => {
+    /* eslint-disable camelcase*/
+    const input = {
+        snake_case: "woot",
+        snakeCase: "oh no", // ensure different value from snake_case for tests
+        camelCase: "fallback",
+    };
+    const snake = new SnakedObject(input);
+    /* eslint-enable camelcase*/
+
+    it('should prefer snake_case keys', () => {
+        expect(snake.get("snake_case")).toBe(input.snake_case);
+        expect(snake.get("snake_case", "camelCase")).toBe(input.snake_case);
+    });
+
+    it('should fall back to camelCase keys when needed', () => {
+        // @ts-ignore - we're deliberately supplying a key that doesn't exist
+        expect(snake.get("camel_case")).toBe(input.camelCase);
+
+        // @ts-ignore - we're deliberately supplying a key that doesn't exist
+        expect(snake.get("e_no_exist", "camelCase")).toBe(input.camelCase);
+    });
+});