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/CMakeLists.txt

@@ -816,6 +816,7 @@ set(SOURCES
     Streams/WritableStreamOperations.cpp
     SVG/AttributeNames.cpp
     SVG/AttributeParser.cpp
+    SVG/Path.cpp
     SVG/SVGAElement.cpp
     SVG/SVGAnimatedEnumeration.cpp
     SVG/SVGAnimatedLength.cpp