Missing attributes in SVGSymbolElement IDL

The [symbol element](https://svgwg.org/svg2-draft/struct.html#SymbolElement) is defined as having the attributes, x, y, width, height, refX, refY. In other elements with these attributes (svg and marker), they correspond to matching IDL attributes, however [SVGSymbolElement](https://svgwg.org/svg2-draft/struct.html#__svg__SVGUseElement__instanceRoot) has no such attributes. These should be added.