Search expression parsing library

Search expressions on the pub site are a bit ad-hoc, and their parsing is will get more and more complex as new requests come in. We'll need a formalized query format with a (separate) query library that does the parsing and exposes the query structure.

This should include:
- grouping expressions: `AND`, `OR`, with `( ... )` sub-groups
- `field:value` expressions
- `"exact match phrases"`
- (optionally) weights