Add gherkin syntax highlighting

[SamyAB]

Add tree-sitter grammar and highlights for the gherkin language aka .feature files.

This is what it looks like with the dracula theme:

This was requested in https://github.com/helix-editor/helix/issues/3115 but it does not completely solve the issue as this PR does not add the LSP support.

[the-mikedavis]

Could you add a license to tree-sitter-gherkin? That will make packaging easier for downstream distributions like Fedora

[SamyAB]

@the-mikedavis I'll do it this evening!

[SamyAB]

@the-mikedavis license added, and languages.toml updated to make the source.rev of the grammar point to the commit with the license

[archseer]

Can you rebase the PR? Looks like the CI was broken at that point in time

[SamyAB]

@archseer rebase done