Add data-driven-styling support for text-font

[nickidlugash]

Data-driven styling for font stacks would be very useful, especially now that expressions have landed. Per chat with @jfirebaugh, this was infeasible before per-tile glyph atlases, but feasibility has not been reviewed since per-tile glyph atlases landed.

We are also now working on expression constraints that will allow the use of data-driven expressions with limited output values.

/cc @anandthakker @aparlato

[jfirebaugh]

This looks relatively straightforward to implement now.

[nickidlugash]

@jfirebaugh that's great! Is there an ETA yet for implementation? Is it blocked by #5393? Please let me know if there's anything I can do to help out.