Add "M<digit>" and "P<digit>" to the default Remarkup ignore list

Summary: Ref T13575. Particularly with the new Apple silicon, I think there are enough domain collisions for M1, M2, P1, etc., to justify adding them to the default ignore list.

Test Plan: Created a mock, then wrote a comment referencing an object on the list (M1) and an object not on the list (T1). Got text and a link respectively.

Maniphest Tasks: T13575

Differential Revision: https://secure.phabricator.com/D21507