Sibling/child items will now remain at the same visual level during
nesting/un-nested, so only the selected item level is visually altered.
Also added new model-based editor content matching system for tests.
Added list nesting support to allow li > ul style nesting which lexical
didn't do by default.
Adds tab handling for inset/outset controls.
Will be a range of edge-case bugs to squash during testing.