Add support for Lark grammar files

[Alhadis]

This PR adds support for Lark grammar files, a variant of EBNF used by the Lark parsing library. The format was brought to my attention by @ThatXliner in Alhadis/language-grammars#4; consult that issue for a more detailed overview of the format.

Checklist:

• I am adding a new language.
  • The .lark extension is used in hundreds of repositories on GitHub:
    • Total search results:
      1,064 indexed and sampled
    • Related usage:
      • 802 files
      • 444 repositories
      • 386 users
    • Unrelated usage:
      • 259 files
      • 3 repositories
      • 2 users
  • I have included samples of real-world usage:
    * ebl_atf.lark: MIT
    * ebl_atf_common.lark: MIT
    * lark.lark: MIT
  • I have included updated a syntax highlighting grammar:
    See Alhadis/language-grammars@32956c6

[lildude]

LGTM