Add workflow to lint Fluent en-US files

[flodolo]

This should prevent surprises when you port changes to mozilla-central

[Snuffleupagus]

r=me, with two more suggestions; thank you!

• Looking at the existing workflows they're all marked as readonly, so can the same be done for the new one? See e.g.

  pdf.js/.github/workflows/ci.yml

  Lines 3 to 4 in fbfacf8

  	permissions:
  	contents: read

• With the Fluent migration all string-ids now start with pdfjs-, could we configure the linter to enforce that?

[flodolo]

With the Fluent migration all string-ids now start with pdfjs-, could we configure the linter to enforce that?

That is not an option currently. The reason is that this linter is typically used in projects with multiple files, so we never implemented this type of check. I'll think about it though ;-)

FIxed also the workflow extension from yaml to yml for consistency with existing workflows.

[flodolo]

I swear I'm usually not this messy in PRs, but that's what happens if I try to multitask 🤦🏼

Filenames should all match now.

[calixteman]

@flodolo thank you for doing this: it was on my todo list but tbh not on my high priority one.