Add Flix language

[sockmaster27]

This adds the Flix programming language.

Flix started as a research project, but is today a fully-featured programming language. It has focused on practical tooling for making the language widely usable, which has helped spark its growing open-source community.

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*.flix
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample sources:
      • https://github.com/mlutze/flixball
      • https://github.com/mlutze/flix-json
    • Sample license:
      • Apache 2.0
  • I have included a syntax highlighting grammar: https://github.com/flix/vscode-flix
  • I have added a color
    • Hex value: #d44a45
    • Rationale: This is the color used in the language's logo.
  • I have updated the heuristics to distinguish my language from others using the same extension.

[sockmaster27]

@lildude
I can see that you've added the "Pending Popularity" tag even though it looks like the extension has over 2k files.
Would you mind sharing what filters you're using or if you're judging by other criteria? It would be helpful to know :)

[lildude]

Sure. https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.flix+NOT+user%3Aflix

I always exclude the language "owner" and other "big hitters" as they often disproportionately affect the results, which is clearly the case here.

[sockmaster27]

That makes sense, thanks!