Support multi cursors

[benjervis]

To add proper multi cursor support, I've upgraded the swap line up/down commands to handle multiple ranges, and have introduced new keybindings to add the extra cursors.

The codemirror file was getting a bit unwieldy with all those key event handlers, so I've split them out into some separate files for organisation.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 34e0c01ba880340b44c996ddd519f774662a9cc5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | -------- | ----- | | playroom | Minor |

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[michaeltaranto]

This is looking sooooo good! I'd like to see some high level integration tests though, like set the code, select next occurrence 4 times, type something, and assert the code in the editor.

Doing this for the various features added would be hugely useful for maintaining all the bindings across code editor implementations.