Add Pyret language

[Nixinova]

• Resolve Add Language: Pyret #6132

Description

Adds the Pyret language, extension .arr.

Checklist:

• I am adding a new language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • .arr extension: 2.6k files total
    • explicitly Pyret: 850 files total, should be over the 200 repo minimum
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • pyret.arr
    • Sample license(s):
      • APACHE2.0
  • I have included a syntax highlighting grammar::
    • Uses samuela/language-pyret, an old version of the recommended grammar repo, because it has a license file and the new repo doesn't.
  • I have updated the heuristics to distinguish my language from others using the same extension.