LibWeb: Implement SVGGradientElement

This is the base class for all SVG gradient types. This supports:

- The `gradientUnits` attribute
- The `gradientTransform` attribute
- And following `xlink:hrefs` for inheriting <stops>/attributes

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -495,6 +495,7 @@ set(SOURCES
     SVG/SVGGElement.cpp
     SVG/SVGGeometryElement.cpp
     SVG/SVGGraphicsElement.cpp
+    SVG/SVGGradientElement.cpp
     SVG/SVGPathElement.cpp
     SVG/SVGCircleElement.cpp
     SVG/SVGEllipseElement.cpp