Implement user cursor and selection tracking

[jonatanklosko]

Closes #59.

Clients broadcast own location (focused cell, cursor position/selection) whenever it changes and we show colorful indicators of that. Additionally the user can follow another one, so that they automatically move around together (:cat: !!).

Design

Whenever user location changes we send it to the LV and broadcast to other clients via the session topic. We then handle the locations accordingly on the client side. Note that as opposed to text deltas, we don't pass/transform the locations via the session process and that's intentional. When it comes to cursor positions we want to make sure the intermediate states when typing look fine and that eventually we show the actual position. Under low latency, transforming cursor positions has no benefits, and under high latency both approaches may show displaced cursor in intermediate states. By simply broadcasting the locations we don't overflow the session process with additional messages and also avoid a ton of complexity.

Also note that if there is just a single user we simply don't send any reports, so there are no implications whatsoever.

Demo

cursors.mp4