Test and stabilize external spell checkers after migration to beforeinput

[oleq]

Let's make sure WSC and Grammarly work OK (but first: how do they work?) when we switch to beforeinput-based typing. There could be some clashes, especially in collaboration features.

[oleq]

• Grammarly used the native DOM API to change the selection and then fires a clipboard event to accept the suggestion. It works fine and seems beyond the scope of beforeinput migration.
• The spell checking in Safari on Mac and Chrome spell checking (independent from Mac's) are handled correctly via insertText and insertReplacementText beforeinput events.
• It looks like the EnterObserver integration with beforeinput might be necessary out-of-the box because sometimes accepting system suggestions using enter creates unnecessary paragraph (Migrate the Enter and ShiftEnter plugins to beforeInput #7999).

[Reinmar]

Closed together with #7462. We're reorganizing the work in #11438.