search: robust keyword scanning

Created by: rvantonder

This PR:

• fixes a bug where we don't always ensure spaces before/after a keyword that requires it (currently and and or)

I'm introducing a helper function to do lookahead and lookbehind for keywords. The lookbehind is avoidable, but a more involved change, so I will address this later (filed in #9812).

• the fix also makes keyword scanning more robust, so that queries like or or or also works
• fixes a bug where parsing would fail if the query only contained whitespace