Add support for parsing more syntax in search queries
Ref T12819. Supports field:term (like title:fulltext) and ~ (substring) and = (exact) operators. Use cases are:
- title:fulltext - Search only in title fields.
- ~substring - Search for term as a substring, not a word.
- title:="exact full title" - Search for results with exactly a given title.
Test Plan: Ran unit tests.
Reviewed By: chad
Maniphest Tasks: T12819
Differential Revision: https://secure.phabricator.com/D18492