LibWeb/SVG: Move path data into Path.{h,cpp}

More things need this than just the `<path>` element, so let's avoid
having to include `SVGPathElement.h` in places that don't need it.

Minor changes at the same time:
- Wrap it in a Path class
- Specify underlying type for PathInstructionType
- Make a couple of free functions into methods
- Give PathInstruction an operator==

No functionality changes.

Libraries/LibWeb/Forward.h

@@ -969,6 +969,7 @@ struct StorageEndpoint;
 
 namespace Web::SVG {
 
+class Path;
 class SVGAnimatedEnumeration;
 class SVGAnimatedLength;
 class SVGAnimatedRect;