fix: new extension for automatically modifying file contents This change implements most of the corresponding proposal as discussed at the 4.4 and 4.6 sprints: https://www.mercurial-scm.org/wiki/AutomaticFormattingPlan This change notably does not include parallel execution of the formatter/fixer tools. It does allow for implementing that without affecting other areas of the code. I believe the test coverage to be good, but this is a hotbed of corner cases. Differential Revision: https://phab.mercurial-scm.org/D2897