search: introduce content: field

Created by: rvantonder

Stacked on #8395.

This adds an optional singleton content: field that overrides the default search pattern when specified. This will be useful for AND/OR queries later, but the immediate win is that it gives a way to express currently inexpressible search patterns in literal mode (addresses #6490).

Note on behavior:

The content:... search patterns are interpreted the same way as they would be along the current search code path for case "file", "path".

This means that the query type:path content:"search" is valid and will return matching file paths for the pattern search. That's not really the intention, but it doesn't look like our existing code has a clean way to separate that casing. I am OK with this until we can restructure the search code, since using content: should be rare, and using it in conjunction with type:path is quite unlikely.