SVGScriptElement needs TT protection too

[lukewarlow]

I know the script enforcement is currently in the HTML integrations section but we need to update it (possibly move the whole section?) to include coverage of the SVGScriptElement. At the very least it needs the new [[ScriptText]] slot to be handled by the parser script enforcement (see #252 )

See also #437

[annevk]

Reopening as #533 got reverted in #565.