GBC Palette Color Functions

[Hawkbat]

There aren't a lot of direct uses for these functions when hand-writing assembly, but it would let the VS Code plugin provide a color picker so it would be a neat thing to add.

• RGB(r, g, b) maps RGB colors to their closest equivalent in the GBC's color space.
• HSV(h, s, v) maps HSV colors to their closest equivalent in the GBC's color space.
• HSL(h, s, l) maps HSL colors to their closest equivalent in the GBC's color space.
• COLORMAP type keyword determines the mapping algorithm used from that line onwards; initial options would include raw, BGB-like, and SameBoy-like presets.