Add Gemini language

[printfn]

Description

Adds the Gemini markup language. No other language uses the .gmi extension, so there shouldn't be any conflicts.

Documentation:

• https://gemini.circumlunar.space/docs/specification.gmi
• https://gemini.circumlunar.space/docs/cheatsheet.gmi

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?q=extension%3Agmi+NOT+nothack&type=Code

    • Here are a few popular repositories with some Gemini documents:

      • https://github.com/skyjake/lagrange/tree/dev/res/about (900 stars)
      • https://github.com/MasterQ32/kristall/tree/master/src/about (540 stars)
      • https://github.com/mbrubeck/agate/blob/master/content/index.gmi (380 stars)
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      https://github.com/michael-lazar/astrobotany/blob/master/astrobotany/static/instructions.gmi (License: ISC)
  • I have included a syntax highlighting grammar: https://github.com/printfn/gemini-vscode
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

.gemini is a long way from meeting our usage requirements (and that sample is massive), however .gmi does meet them.

If you remove the .gemini extension (and massive sample), we can merge this PR and peeps can use an override until the extension gains enough usage to be added.

[printfn]

Thanks for the review! I've removed the .gemini extension now.