Add drawable attribute to SVGElement

The HTML -in-Canvas APIs make use of an attribute, drawable, on elements in the canvas subtree to indicate those elements need special treatment to support drawing them into a canvas. The HTML spec PR currently puts that attribute on HTMLElement but to support all content we would like to put it on HTMLOrSVGOrMathMLElement.

This issue is seeking SVG WG approval to add the drawable attribute to HTMLOrSVGOrMathMLElement.