Add constants for color shades

[Rangi42]

From the BGP docs, 0 = white, 1 = light, 2 = dark, 3 = black. I sometimes mix these up and have to look up the info (or, of course, just use named constants).

Even "Pseudo-ops Considered Harmful" recommends an lb macro, e.g. for lb hl, $12, $34:

MACRO lb
    assert -128 <= (\2) && (\2) <= 255, "Second argument to `lb` must be 8-bit!"
    assert -128 <= (\3) && (\3) <= 255, "Third argument to `lb` must be 8-bit!"
    ld \1, (LOW(\2) << 8) | LOW(\3)
ENDM

"lc" (load crumbs) or "dc" (define crumbs) can also be useful, mainly for DMG colors, e.g. lc a, SHADE_WHITE, SHADE_LIGHT, SHADE_DARK, SHADE_BLACK.