Run mesher in web worker

[deathcap]

To improve responsiveness, meshing should run asynchronously in a web worker.

An individual chunk can be remeshed quickly enough to not cause any noticeable lag (required to allow interactive mining and building), but it becomes apparent when exploring and generating new chunks. Terrain generation https://github.com/deathcap/voxel-land runs in a worker, so the pause is likely caused by the chunk meshing (to do: confirm with profiler).

ref #4