Create an OOUI editor widget

For now this exists in parallel to the old editor system,
and nothing uses it yet. We'll migrate things in follow-up
commits until the old system is unused and can be removed.

Notable changes:

• Warning for anonymous editors is no longer a tooltip on the side, but now appears above the editor
• Rename editor preference value 'none' to 'wikitext', and provide b/c logic in both directions
• Add b/c logic in SwitchEditorAction
• New VE integration uses one Target instance that is reused, rather than creating a new one every time
• New system keeps focus when switching between editors

Bug: T103572
Bug: T97290
Bug: T101189
Change-Id: Ic2353ef6e9155bd921d224d129c7b70cb22ba8a7