A custom property declaration that is the last non-comment child but is
followed by comment siblings was stringified without its terminating
semicolon. Unlike a normal declaration, a custom property keeps everything
up to the next `;` or `}` as its value, so on re-parse the trailing
comments were folded into the value and the comment nodes disappeared.
Building such a tree with append()/insertAfter()/after() therefore
silently dropped the comments. Emit the semicolon when a custom property
still has following siblings so the output round-trips, mirroring the
existing handling for childless at-rules.
