symset S_darkroom, S_vibrating_sqaure

symset S_darkroom, S_vibrating_sqaure

Symbol set parsing rejected S_darkroom and S_vibrating_square.
Now it will accept them.

The fact that this bug wasn't noticed indicates that none of the
3.6.0 symbol sets (other than "Default symbols") is specifying a
value for either of these symbols.

This also changes the default vibrating square value from yellow
'^' (caret) to purple '~' (tilde).  I don't think there's any risk
of mistaking it for a long worm tail (brown '~') and it emphasizes
that it isn't really a trap.