the buffer search and project search bars both already load the `regex`
language and put it on their query buffer when the regex filter is on.
the text finder never got that, so a regex you type in there is just
plain text.

did the same thing here. the only slightly annoying part was that the
picker's head editor is `pub(crate)`, so text_finder couldn't get at it.
added a small `query_editor()` accessor to `Picker` for that.
`adjust_query_regex_language` mirrors the two existing ones.

test asserts the query buffer's language is regex with the filter on,
and gone once it's off.

Closes #59945.

Release Notes:

- Improved the text finder by highlighting the query as a regex when the
regex filter is on

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Co-authored-by: Kirill Bulatov <kirill@zed.dev>