search: make query concat transformer return []Node not Pattern
Created by: rvantonder
Setup for RFC 675 and new pattern interpretation. Need this to implement combination of AND
ing and concatenating patterns. A branch on top of this uses this interface and works, but I haven't cleaned that up/tested fully yet. Will do next, but this is ready to ship.
Context in the long version: https://github.com/sourcegraph/sourcegraph/pull/37658#discussion_r906441195
Test plan
Semantics-preserving. Runs against existing tests.