Simply resets all of the palette colors to their defaults.

We discussed this on the original repo (dan200/ComputerCraft#343), and instead settled on a term.nativePaletteColour, which just acts like term.getPaletteColour but returning the default.

This is more flexible than a reset method, and means such a thing can be implemented as follows:

for i = 0, 15 do term.setPaletteColour(2^i, term.nativePaletteColour(2^i)) end