Add mdsvex

[mahtaran]

Adds support for the mdsvex language, a Markdown preprocessor for Svelte. At the time of writing, a search returns 4.1k mdsvex files hosted on GitHub: https://github.com/search?q=path%3A*.svx+NOT+%2F%5CA%28%3B%7C%5C*begin%5Cs%7C%5C*units%5Cs%29%2F+NOT+is%3Afork&type=code

Description

No grammar or LSP exists yet. Progress for these is tracked in pngwn/MDsveX#121
For code editors, files can best be marked as Markdown.

Sample is taken from sveltesociety.dev, licenced under the MIT License.

Colour was chosen to be identical to the primary accent colour of the mdsvex website

Note that the .svx extension is also used for Survex data files. Since that is currently not supported, no heuristic was added. Should the need to add such a heuristic arise in the future, the following RegEx will match for Survex data files: /\A(;|\*begin\s|\*units\s)/

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*.svx+NOT+%2F%5CA%28%3B%7C%5C*begin%5Cs%7C%5C*units%5Cs%29%2F
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/svelte-society/sveltesociety.dev/blob/d2f08cc604a205e51aa8cae3856aae64498096d2/src/routes/recipes/stores/%2Bpage.svx
    • Sample license(s):
      • MIT License
  • I have included a syntax highlighting grammar
    No grammar exists yet, progress is tracked in VS Code syntax highlighting? File Type Icon? pngwn/MDsveX#121.
    For code editors, files can best be marked as Markdown (see linked PR).
  • I have added a color
    • Hex value: #5f9ea0
    • Rationale: primary accent colour of the mdsvex website
  • I have updated the heuristics to distinguish my language from others using the same extension.
    Extension is also used for Survex data files,
    however Linguist currently doesn't support those, so no heuristic was added.
    Should the need for one arise in the future, the following RegEx will match Survex data files:
    /\A(;|\*begin\s|\*units\s)/

[lildude]

Looks like Survex is popular enough for inclusion so please add it and the heuristic in this PR to ensure they don't get incorrectly classified.

[mahtaran]

(Renamed branch to clarify addition of Survex, will create a new PR shortly)

[lildude]

You didn't need to. We don't care about branch names or clean history as everything is squashed on merge.

[mahtaran]

Ah, I see. To be completely honest, I had expected GitHub's "rename branch" functionality would also update the base branch of this PR, instead of closing it. New PR is #7194, apologies for the additional effort.