Replaced the standing sign (90 degrees) starting replacement node for on_rotate().
Tested on all signs, testing all degrees of rotation (all 16 of them).
fixes newly found bug: non-oak standing sign becomes oak standing sign when initially rotated with a screwdriver.
Has compatibility layer for signs_dark, the original signs, etc.
Has added greyscale sign for making your own signs. :)
TODO: add in translations, tweak the sign colors.