Problem:
Previously, the fallback logic to ".conf" was located outside of
`vim.filetype.match()` and directly within the AutoCmd definition. As a
result, `vim.filetype.match()` would return nil instead of ".conf" for
fallback cases (#30100).
Solution:
Added a boolean return value to `vim.filetype.match()` that indicates
whether the match was the result of fallback. If true, the filetype will
be set using `setf FALLBACK <ft>` instead of `setf <ft>`.
e2cb675705