LibWeb: Support x and y attributes on nested SVGs

This allows positioning a child SVG relative to its parent SVG.

Note: These have been implemented as CSS properties as in SVG 2, these
are geometry properties that can be used in CSS (see
https://www.w3.org/TR/SVG/geometry.html), but there is not much browser
support for this. It is nicer to implement than the ad-hoc SVG
attribute parsing though, so I feel it may make sense to port the rest
of the attributes specified here (which should fix some issues with
viewport relative sizes).

Tests/LibWeb/Layout/input/svg/svg-inside-svg-with-xy.html

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg"
+  xmlns:xlink="http://www.w3.org/1999/xlink">
+  <svg x="10">
+    <rect x="10" y="10" height="100" width="100"
+        style="stroke:#ff0000; fill: #0000ff"/>
+  </svg>
+  <svg x="200" y="15">
+    <rect x="10" y="10" height="100" width="100"
+        style="stroke:#009900; fill: #00cc00"/>
+  </svg>
+</svg>