Enforce warnings for unsupported parameters and filters in structural search

Created by: rvantonder

The documentation covers that some of the search parameters are unsupported. Specifically, things like case (structural search is always case sensitive) and -lang and multiple lang options just don't make sense contextually.

The code currently just ignores these options, and as a quality measure I want to enforce and warn/alert on any such usages. Note that this is aligned with the higher level goals outlined in RFC 75, where we separate and explicitly define and enforce how Sourcegraph's parameters interact with various kinds of search.