Problem: Language names are only registered for filetype<->language
lookups when parsers are actually loaded; this means users cannot rely
on `vim.treesitter.language.get_lang()` or `get_filetypes()` to return
the correct value when language and filetype coincide and always need to
add explicit fallbacks.
Solution: Always return the language name as valid filetype in
`get_filetypes()`, and default to the filetype in `get_lang()`. Document
this behavior.
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