earcut: package stability, or move it to the graveyard

[emidoots]

This is a tracking issue for declaring the mach-earcut package as stable. This means:

1. It is in use inside Mach engine and we feel good about it overall.
2. None of the outstanding issues are of major concern.
3. The README and website have been updated to remove the experimental warning.

Why remove it? It's a port of mapbox' earcut, which although it is the best that is actually open source, is not perfect. Almost no tesselators are (see the mach-earcut docs for an explanation). There have been suggestions that https://github.com/nelipuu/tessellate has better correctness, but would be much slower (so in practice, you may want the option to use both.) -- but ultimately it's unclear if we need a polygon tessellator as part of Mach for the gkurve work or not.

[emidoots]

Moved to https://github.com/slimsag/mach-earcut - to signal this is no longer an official Mach project.