search: rename `repo:contains` predicates to be consistent

Created by: tbliu98

Part of #39767 (closed)

As described in the linked issue, this PR:

• Renames repo:contains(file:a content:b) to repo:contains.file(path:a content:b)
• Renames repo:contains.file(a) to repo:contains.path(a)
• Removes support for repo:contains(file:a content:b)

Note that if we think keeping repo:contains around is actually the way to go, it will be easy to add it back.

This PR includes corresponding frontend changes to syntax highlighting, hovers, and the search reference panel.

Test plan

Updated unit tests. Integration tests. Manually tested hovers/suggestions/syntax highlighting/reference panel