scripts: replace dark-variant.sh with go scripts/variants

uses regex mostly, resulting in a significantly faster execution (old:
~2s, new: ~31ms). Only matches classList.add/remove and class="..."
(won't touch string literal variables or colours), but these weren't
really used anyway. Supports multi-line classList.add/remove, which was
the reason I wrote this anyway (formatting the codebase introduced some
of these).

.git-blame-ignore-revs

@@ -0,0 +1 @@
+817107622a8fe6f2fdaf198da4b2632854aa9bac