fix(flowchart): paint the edge label background behind math

An edge label with $$...$$ has each line wrapped in a <div> instead of
a <p>, so it missed the `.edgeLabel p` background rule. All that was
left behind it was the faded `.labelBkg`, so the edge showed through
the label. Give those wrappers the same background as the <p>.

Resolves #5543

.changeset/flowchart-math-edge-label-background.md

@@ -0,0 +1,5 @@
+---
+'mermaid': patch
+---
+
+fix: paint the edge label background behind math in flowchart edge labels so the edge no longer shows through them