macOS text substitutions: Add the Edit → Substitutions menu.

Toggling substitution types via this menu is temporary, which is
consistent with other Mac apps' behavior.

Bug: 42434
Change-Id: Ifa07909464f8a10f1e77be176fc456eb8855f093
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1654188
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670241}