Add 2bpp fonts

[riknoll]

Currently Arcade only has support for 1bpp and 4bpp images. 4bpp is probably a little too much for a font, but 2bpp would work nicely. This will need to be implemented in pxt-common-packages.

This will also require adding multiple colors to https://riknoll.github.io/font-editor/

...and adding multiple colors to the set/get color blocks.