Expose ${BATCH_ID} as a built-in variable for the styling language

[lilleyse]

The shading language could support a built-in variable ${BATCH_ID} that returns the feature's batch id. This is similar to how point clouds have ${POSITION}, ${COLOR), and $(NORMAL) for getting the point's internal attributes. ${BATCH_ID} would work for any tile type.

This isn't a critical thing to add, it's mainly a convenience.

[pjcozzi]

Labeled next; not needed for 1.0.

[lilleyse]

Noted in #2 (comment)