search: search pattern partitioning for and/or queries

Created by: rvantonder

To process and/or expressions for content, we need to separate search patterns expressions from the parameters that scope them (e.g., file:, repo:). This PR adds a helper function PartitionSearchPattern to do that. The function performs rudimentary validation to block certain queries that we won't be able to perform just yet. It is rudimentary because it doesn't cover every query that may match a user's intuition re ambiguity.