Undo undoes the initial selection [Minor]

[PaddyMann]

V minor bug:

• When we load the slate editor, we focus on the editor using:

  useEffect(() => {
  ReactEditor.focus(editor);
  }, []);

• With the undo in slate-collaborate, pressing undo will deselect the editor.

• This differs to the Slate's withHistory plugin, which will leave the editor selected.

[wleroux]

The current design is to undo any changeset (selection included). Changing the design to undo changes only if there is a mutation to the value is a simple enhancement to the existing design. Will do!

[wleroux]

Completed in 0.0.21