Query validation of Sourcegraph language parameters

Created by: rvantonder

The and/or parser introduces a new way to parse parameters and search patterns. We need to validate general properties:

• Valid regular expression values for filters like repo:...
• Valid values for filters like case:...

These parameters are (mostly) independent of searching for content in files, and validation remains the same for things like repo, commit, diff, symbol, and filepath searches.

Understand more about the Sourcegraph language mentioned in RFC 75