Make color font support more obvious

[ctrlcctrlv]

It isn't obvious that SILE supports color fonts. If you load a font with COLR/CPAL table, that could be output as color, SILE should try to help you more than making you ask Simon on Twitter and being directed to packages/color-fonts.lua :-)

[alerque]

Agreed. This is something we should be able to detect and load the necessary support package automatically just by specifying the font. It just needs doing.

[simoncozens]

This and #936 are actually very similar: when a font is loaded, see what it can do, and respond appropriately.