"Smooth Rainbow" magic tool (variant of "Rainbow")

Numerous people commented on Twitter that a piece of artwork
("Chromie Squiggle #7583", created in "ArtBlocks") looked like it
was made in Tux Paint.

However, the classic (2002-era) "Rainbow" tool cycled coarsely
through solid colors.  This adds a gradient.

magic/magic-docs/en/rainbow.txt

@@ -4,3 +4,5 @@
 
    This is similar to the paint brush, but as you move the mouse around, it
    cycles through a spectrum of bright colors.
+
+   See also: Smooth Rainbow