Added Nickel language

[mkatychev]

Description

This PR adds support for the Nickel and attempts to resolve #6503

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*.ncl+nickel
  • I have included a real-world usage sample for all extensions added in this PR:
  • Sample source(s):
    • https://github.com/tweag/nickel/blob/dbd52ac010199a4b391b3c9fa8bec000534d331a/examples/imports/imports.ncl
    • https://github.com/tweag/nickel/blob/dbd52ac010199a4b391b3c9fa8bec000534d331a/examples/arrays/arrays.ncl
    • https://github.com/tweag/nickel/blob/dbd52ac010199a4b391b3c9fa8bec000534d331a/examples/config-gcc/config-gcc.ncl
  • Sample license(s):
    • https://github.com/nickel-lang/organist?tab=MIT-1-ov-file
  • I have included a syntax highlighting grammar: https://github.com/tweag/nickel/tree/master/lsp/vscode-extension
  • I have added a color
    • Hex value: #e0c3fc
    • Rationale: Colour of Nickel logo
      
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Important

The changes in this PR will not appear on GitHub until the next release has been made and deployed. See here for more details.