Search: add support for excluding terms

It looks like Core supports excluding terms: https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-query.php#L1337

I guess this means that we need to do something also even though no one ever uses that syntax. This probably needs more data, but adding an issue to track it.

Start with the linked WordPress Core WP_Query reference and compare its excluded-term syntax with Jetpack's search implementation. Gather usage or search-result examples before deciding the expected behavior; the work is done when the supported syntax and resulting exclusions are documented by tests.

Written by the indexing model from the issue text.