New script text associated data and associated mechanisms need adding to SVGScriptElement

[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 )

An SVGScriptElement is also a Node descendent so might need handling in the Node.textContent code

See also #437