Create numbered and bulleted lists

This PR aims to add the creation of lists in a document. This only aims to support numbered and bulleted lists and not image bullet lists for now.

# TODO
* [x] Editor UI buttons
* [x] Detect list style at selection
* [x] Ops for list insertion/removal
* [x] OT support
* [x] Convert selected text to a list or selected list back to text
* [x] Add feature flags and session constraints
* [x] Default styles for numbered and bulleted lists

Hrm. This one is tricky. The comment is incorrect though, as no merging or splitting is necessary for this condition.

The hard part is that on a merge, the ``RemoveList`` start position actually moves *down* to the next paragraph following the freshly merged one. But, the op does not provide enough detail to figure this out...