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/...`.

scripts/release/pre-release.sh

@@ -12,6 +12,3 @@ do
         jq ".$i = .matrix_lib_$i" package.json > package.json.new && mv package.json.new package.json && yarn prettier --write package.json
     fi
 done
-
-# Ensure that "type": "module" is present
-jq '.type = "module"' package.json > package.json.new && mv package.json.new package.json && yarn prettier --write package.json