Problem: Injecting languages for file redirects (e.g., in bash) is not
possible.
Solution: Add `@injection.filename` capture that is piped through
`vim.filetype.match({ filename = node_text })`; the resulting filetype
(if not `nil`) is then resolved as a language (either directly or
through the list maintained via `vim.treesitter.language.register()`).
Note: `@injection.filename` is a non-standard capture introduced by
Helix; having two editors implement it makes it likely to be upstreamed.
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