Run l10n string extractor on pre-commit

Follow up to https://github.com/mdn/fred/pull/452#pullrequestreview-3931216678

A naive implementation (run npm run l10n && git add l10n/template.ftl) has its issues, I tested it locally with:

• add string to components/a.js
• add string to components/b.js
• stage a.js
• commit
• new strings from both a and b end up in template.ftl

The extractor script could be updated to take a list of files to operate on, however that doesn't currently work either because it doesn't read template.ftl but regenerates and overwrites it in a deterministic way - running on only a few files would mean only a few strings would end up in the output.

Perhaps the easiest approach is to run it as a rspack plugin on every build - like we do already with the custom element types file generation - and leave it to the developer to commit whichever parts of it are relevant in a multi-commit clean-stage messy-tree workflow?

Start with the implementation behind npm run l10n and the existing custom element types generation used in the rspack build. Determine a workflow that preserves the complete deterministic l10n/template.ftl while handling staged and unstaged files; done means the process updates the template without dropping strings from other components.