Add missing loadAfter voxel-registry in lib/blocks plugin
Change registry and stitch plugins to proper dependencies ('built-in' plugins)

Makes more sense to include voxel-registry and voxel-stitch as actual
dependencies, instead of letting the caller provide them, since
game-shell-voxel explicitly uses them (in gl-init handler, setting up
blocks and textures). Introduces concept of BUILTIN_PLUGINS. Callers can
provide custom options to these plugins, or they default to '{}' (but
they cannot currently override these plugins with their own versions).
Add list of meshes