Implement the editing/parsing/layout routines in a WebWorker

[capnmidnight]

STATE! State is difficult to manage! But it's expensive to recompute state on every frame of rendering/every edit.

After Issue #160 was finished, edits to large documents are pretty fast. But big changes are still pretty slow. They're "fast" in the sense that it's faster than VS Code or other JS-based text editors, but it's "slow" when regarding the needs for VR.

Putting all the logic in a WebWorker, off the UI thread, could at least hide that slowness from the user.