inject language based on file extension & shebang (#3970)

* inject language based on file extension

Nodes can now be captured with "injection.filename". If this capture
contains a valid file extension known to Helix, then the content will
be highlighted as that language.

* inject language by shebang

Nodes can now be captured with "injection.shebang". If this capture
contains a valid shebang line known to Helix, then the content will
be highlighted as the language the shebang calls for.

* add documentation for language injection

* nix: fix highlights

The `@` is now highlighted properly on either side of the function arg.

Also, extending the phases with `buildPhase = prev.buildPhase + ''''`
is now highlighted properly.

Fix highlighting of `''$` style escapes (requires tree-sitter-nix bump)

Fix `inherit` highlighting.

* simplify injection_for_match

Split out injection pair logic into its own method to make the overall
flow easier to follow.

Also transform the top-level function into a method on a
HighlightConfiguration.

* markdown: add shebang injection query

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