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).

Userland/Libraries/LibWeb/CSS/Properties.json

@@ -2196,6 +2196,32 @@
       "normal"
     ]
   },
+  "x": {
+    "__comment": "This is an SVG 2 geometry property, see: https://www.w3.org/TR/SVG/geometry.html#X.",
+    "inherited": false,
+    "initial": "0",
+    "valid-types": [
+      "length [-∞,∞]",
+      "percentage [-∞,∞]"
+    ],
+    "percentages-resolve-to": "length",
+    "quirks": [
+      "unitless-length"
+    ]
+  },
+  "y": {
+    "__comment": "This is an SVG 2 geometry property, see: https://www.w3.org/TR/SVG/geometry.html#Y.",
+    "inherited": false,
+    "initial": "0",
+    "valid-types": [
+      "length [-∞,∞]",
+      "percentage [-∞,∞]"
+    ],
+    "percentages-resolve-to": "length",
+    "quirks": [
+      "unitless-length"
+    ]
+  },
   "z-index": {
     "affects-layout": false,
     "affects-stacking-context": true,