Add XHAST linter standards

Summary: Ref T8742. As mentioned in D13512. This still needs some work, but looks roughly how I expect it to. Mainly, I want to move the standards stuff to the linter itself rather than the linter rule. I wanted to push this out for some initial feedback though.

Test Plan: This still needs work.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T8742

Differential Revision: https://secure.phabricator.com/D13942