fix removeMark

[BrianHung]

Removing marks broke with #8, specifically within the handleMark function.

d1abb5c#diff-1cdd5cad54822249d22829cef95393e37707fcdb58e789e1a217c9f847ff9d9aL114-L120

This PR reimplements removing marks by checking if mark value is null, which is the only way we know that a mark is deleted as the MarkPatch type is not that expressive (Peritext has separate AddMark and RemoveMark patches).

type MarkPatch = {
	action: 'mark';
	path: Prop[];
	marks: am.Mark[];
};

Slightly awkward with ProseMirror's API, as you can create a mark passing in null (defaultAttrs is used to fill in missing attrs).

Fixes #12

[BrianHung]

Automerge does seem to have an UnmarkPatch., but it is also currently unused.

I would expect this to be used when you call Automerge.unmark(). But in @automerge/[email protected], I instead get a MarkPatch with value null.