[Feature] Support facet grammar

[hunmix]

What problem does this feature solve?

Datajoin is not supported for group mark in vgrammar. It is quite inconvenient to map data to nested mark structure. Glyph mark cannot be nested so it won't help when a complicated mark structure need to be mapped. It will be much better if vgrammar provide with facet function for group mark

What does the proposed API look like?

Waiting for facet grammar design.