PSA: When landing new ES modules into the tree please use the
`.mjs` file extension. For system ES modules, please continue
using the `.sys.mjs` extension.
This also turns on some other rules for the module files which are useful to have, for example, checking for no-unused-vars on the global scope.
If you have a specific case where for some reason `.mjs` can't be
used, please have a chat in #lint:mozilla.org (Lint and
Formatting) on Matrix and we can figure out an appropriate
There are some pre-existing cases of ES module files in the tree that still use `.js`, which we are working towards changing.