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 `` to a `` for example.

No file or test is named in the issue. Start by locating the SVG DOM declarations and the existing appendChild overload patterns; done means the relevant SVG elements reject invalid child types such as on while accepting their permitted children.

Written by the indexing model from the issue text.