Added multi-cursor support

[ivan-kzn]

added support of sharing multiple cursors via awareness messages

[dmonad]

This looks fine. But I ask that you find somebody else who can test that this actually works in different environments (Chrome, Firefox, Safari).

Is getSelections available in all browsers? As far as I know, most browsers don't support multi-selections.

One thing that I'm hesitant about is that this PR breaks compatibility with y-codemirror (in theory, you can currently sync content and awareness information with a codemirror instance). But I don't think this should be a blocker, unless someone finds a good reason.

[ivan-kzn]

ok, I'll test this functionality in browserstack and get back with results. btw we haven't met such issues for now