Add Gleam language

[Aloso]

Description

Adds the Gleam language, including syntax highlighting support. Gleam is a modern, functional language that can compile to Erlang/BEAM code or to JavaScript.

See gleam-lang/gleam#477

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3Agleam+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/gleam-experiments/crypto/blob/main/src/gleam/crypto.gleam
      • https://github.com/gleam-lang/stdlib/blob/main/src/gleam/dynamic.gleam
      • https://github.com/gleam-experiments/snag/blob/main/src/snag.gleam
      • One of the samples I wrote myself
    • Sample license(s): Apache 2.0
  • I have included a syntax highlighting grammar: Preview in Lightshow
  • I have updated the heuristics to distinguish my language from others using the same extension.
    • doesn't seem necessary since the file extension isn't used elsewhere.

[lpil]

Oh dear! We have both made a PR for Gleam #5688

I should have made it clearer that I'd made one already, sorry about that.