cmd/compile: add a dark mode to ssa html generation which can be toggled

add a tag that when clicked, toggles a dark mode. It keeps intact
the grayed out dead values/blocks, all the highlight colors, and ensures
text is always readable.

Fixes #34325

Change-Id: I4af1e4b5f4a5b63e54c992e90f8474cc51c63465
Reviewed-on: https://go-review.googlesource.com/c/go/+/220260


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>