Add FIRRTL language

[seldridge]

Add FIRRTL [1], a language/IR that is produced by Chisel [2] and compiled to Verilog using the CIRCT [3] project.

Description

This adds the FIRRTL language.

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?type=code&q=NOT+is%3Afork+path%3A*.fir+circuit
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • LFSR.fir was generated using the Chisel LFSR library.
      • Memory.fir is adapted from memoryMetadata.fir.
    • Sample license(s):
      • The generated code should be licensed in the same way as other code in Linguist as it is a novel contribution, though based on a real code.
      • The Memory.fir example is from llvm/circt which is Apache 2 licensed.
  • I have included a syntax highlighting grammar: https://github.com/chipsalliance/firrtl-syntax
  • I have added a color
    • Hex value: #2f632f
    • Rationale: This is the same color used for the FIRRTL "tree" logo: https://raw.githubusercontent.com/freechipsproject/firrtl/master/doc/images/firrtl_logo.svg?sanitize=true
  • I have updated the heuristics to distinguish my language from others using the same extension.

I think this is over the bar for a new language. GitHub is showing 2.8k files and there appear to be a sufficiently large number of individual repositories. I've tried filtering out projects which have large numbers of these files, e.g., llvm/circt, and things still look good. The following query removes all llvm, chipsalliance, or ucb-bar users and is still showing 2k+ samples:

https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.fir+circuit+NOT+user%3Allvm+NOT+user%3Achipsalliance+NOT+user%3Aucb-bar

This pre-emptively adds a heuristic to avoid a collision with .fir which is also used by Fortran IR. Fortran IR is not presently in Linguist. Fortunately, the heuristic to avoid collision is trivial. Let me know if the PR should not pre-emptively include this heuristic. Heuristics should be unnecessary if there is no other language using the extension. This may need to revisited if Fortran IR wants to add syntax highlighting.

Metadata

Fixes chipsalliance/firrtl-spec#54.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.