Add Effekt language support

[marvinborner]

Add Effekt language support with .effekt extension and its literate variant with the .effekt.md extension.

Description

This PR adds support for Effekt as a new programming language in Linguist. Effekt is a research language with lexical effect handlers and lightweight effect polymorphism. Being in active development since over 5 years, by now it amounted to considerable research contributions as well as adoption in projects experimenting with effects and handlers.

I believe that the current interest in the language as well as its adoption is enough to meet Linguist's requirements. Literate Effekt via .effekt.md of course does not match the adoption of .effekt.

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*.effekt
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.effekt.md
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • inference.effekt: probabilistic inference
      • parser.effekt: lexer + parser
    • Sample license(s): MIT
  • I have included a syntax highlighting grammar: https://github.com/effekt-lang/effekt-vscode
  • I have added a color
    • Hex value: #83488d
    • Rationale: It is used in the color scheme on https://effekt-lang.org/
  • I have updated the heuristics to distinguish my language from others using the same extension.
    • Effekt uses .effekt exclusively.

[lildude]

.effekt isn't quite there for our usage requirements, but doesn't appear to be far off, hence adding the "Pending popularity" label.

.effekt.md is a very very long way off and it's inclusion in this PR will block merging until it meets our requirements. The samples are also too big… if the file is suppressed in the diff, it's too big. I recommend removing "Literate Effekt" for now and submit a PR later when it meets our usage requirements.