Some SVG types would benefit from stricter appendChild() overloads

``` typescript interface SVGFEComponentTransferElement { appendChild(newChild: SVGFEFuncAElement): SVGFEFuncAElement; appendChild(newChild: SVGFEFuncBElement): SVGFEFuncBElement; appendChild(newChild: SVGFEFuncGElement): SVGFEFuncGElement; appendChild(newChild: SVGFEFuncRElement): SVGFEFuncRElement; appendChild(newChild: Comment): Comment; } interface SVGFEMergeElement { appendChild(newChild: SVGFEMergeNodeElement): SVGFEMergeNodeElement; appendChild(newChild: Comment): Comment; } ... ``` to override the generic appendChild(Node) inherited from Node, and so on. This would make it an error to append a `<g>` to a `<feMerge>` for example.