Implement "arc work", to replace "arc feature"

Summary: Ref T13546. Fixes T2928. Adds a new "arc work" workflow which functions like the older "arc feature" workflow, but with modern infrastructure.

Test Plan: Used "arc work" to begin work on branches, bookmarks, and revisions in Git and Mercurial.

Maniphest Tasks: T13546, T2928

Differential Revision: https://secure.phabricator.com/D21336