Code ownership: Register file:has.owner() predicate

Created by: philipp-spiess

Closes #38937 (closed)

As a first step towards bringing the code ownership concepts into our core workflows, we're registering a new predicate that we can later use to filter results based on ownership (c.f. https://github.com/sourcegraph/sourcegraph/issues/38148#issuecomment-1183579162).

The current consensus seems to be to use a file predicate, the syntax would look like this:

file:has.owner(@philipp-spiess)

(cc @muratsu @jjinnii @ryankscott for syntax feedback?)

Test plan

Added a test:

ok  	github.com/sourcegraph/sourcegraph/internal/search/query	0.259s

The predicate does not have an implementation so this will currently look for filenames with has.owner(@philipp-spiess) in it's path AFAICT.