LibWeb: Add SVG <polygon> element and test case :^)

Author: https://github.com/AtkinsSJ
Commit: 2fad940b0b
Pull-request: https://github.com/SerenityOS/serenity/pull/12444
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/SVG/SVGPolygonElement.idl

@@ -0,0 +1,5 @@
+[Exposed=Window]
+interface SVGPolygonElement : SVGGeometryElement {
+};
+
+// SVGPolygonElement includes SVGAnimatedPoints;