Add support for parsing more syntax in search queries

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18492