Add `.peggy` for PEG.js

[spenserblack]

Description

PEG.js seems to be abandoned, and peggy is the new fork that the community is migrating to. They've adopted .peggy as their extension, but the grammar is very close, if not identical, to PEG.js.

In my popularity search I stumbled on a Go project that seems to combine PEG and Go code in one file. There is also this Haskell project.

Checklist:

• [x] I am adding a new extension to a language.
  • [x] The new extension is used in hundreds of repositories on GitHub.com (it looks like it's kind of borderline right now, maybe not popular enough yet)
    • Search results for each extension:
      • https://github.com/search?q=path%3A*.peggy+NOT+owner%3Apeggyjs+NOT+is%3Afork+NOT+owner%3Acountnazgul+NOT+owner%3Ayy0931+NOT+owner%3Amscgenjs+NOT+owner%3Atanakh+return&type=code
  • [x] I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • ~~semver.peggy~~
      • abnfp.peggy
    • Sample license(s): Apache-2.0
  • [ ] I have included a change to the heuristics to distinguish my language from others using the same extension.

[spenserblack]

One of the maintainers of Peggy was interested in updating the grammar: https://github.com/peggyjs/peggy/pull/544#issuecomment-2315524798

Would it be best to do that in a separate PR?

[lildude]

Would it be best to do that in a separate PR?

Yes. Please do it in a new PR. Be quick. I plan to start the next release tomorrow with the aim of rolling it out on Monday.