Multi-cursor support

[PEZ]

As we are in the process of adding multi-cursor support to Calva Paredit (spearheaded by @riotrah) I can recommend taking this on early. We have it working for selections and movement and it is a major productivity boost.

[simonacca]

Very interesting, thanks for the tip! At the moment it's not clear to me even what the desired behavior for most commands, will take a look at the Calva implementation for inspiration :)

[riotrah]

Yeah seriously - it's possibly going to be much harder to add multi cursor later vs building everything with it in mind!

[riotrah]

This giant old PR will have more examples of what multicursor might look like for each type of edit/movement etc. Namely in the unit tests.

BetterThanTomorrow/calva#1606