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

Author: https://github.com/AtkinsSJ
Commit: 116a1f485c
Pull-request: https://github.com/SerenityOS/serenity/pull/12444
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/Forward.h

@@ -251,6 +251,7 @@ class SVGGeometryElement;
 class SVGGraphicsElement;
 class SVGLineElement;
 class SVGPathElement;
+class SVGPolylineElement;
 class SVGRectElement;
 class SVGSVGElement;
 }
@@ -460,6 +461,7 @@ class SVGGeometryElementWrapper;
 class SVGGraphicsElementWrapper;
 class SVGLineElementWrapper;
 class SVGPathElementWrapper;
+class SVGPolylineElementWrapper;
 class SVGRectElementWrapper;
 class SVGSVGElementWrapper;
 class TextEncoderWrapper;