API: add SVGUnknownElement (#2263)
API: add SVGUnknownElement (#2263)

* SVG/Elements: add <unknown>

Sources:
1. Scalable Vector Graphics 2 specification:
   https://svgwg.org/svg2-draft/struct.html#InterfaceSVGUnknownElement
2. Chromium repository:
   https://github.com/chromium/chromium/blob/f18e79d901f56154f80eea1e2218544285e62623/third_party/WebKit/LayoutTests/external/wpt/svg/struct/UnknownElement/interface-expected.txt
3. Firefox bug tracker:
   https://bugzil.la/589640
   https://bugzil.la/840417
   https://bugzil.la/1239218
4. Firefox repository:
   https://github.com/mozilla/gecko-dev/blob/faeb9b7dddf16e5d3dc18c22f16b1106e4eed1b2/testing/web-platform/meta/svg/interfaces.html.ini#L122

* rewrite SVG/Elements/unknown as API/SVGUnknownElement

* API/SVGUnknownElement: assume no support for Edge/IE/Safari

See: https://github.com/mdn/browser-compat-data/pull/2263#issuecomment-402707720

mdn/browser-compat-data,teoli2003/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data