* 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/...`.
* Declare matrix-js-sdk as an ES module
* Rename `babel.config.js` to show it is a CommonJS module
... otherwise it gets broken by `scripts/switch_package_to_release.js`
