LibWeb: Parse the content property

For now, we only understand `none`, `normal`, `<image>` and `<string>`.
The various other functions and identifiers can be added later.

We can *almost* use a StyleValueList for this, except it's divided into
two parts - the content, and the optional "alt text". So, I've added a
new StyleValue for it.

Userland/Libraries/LibWeb/CSS/Properties.json

@@ -510,6 +510,18 @@
       "hashless-hex-color"
     ]
   },
+  "content": {
+    "inherited": false,
+    "initial": "normal",
+    "__comment": "FIXME: This accepts a whole lot of other types and identifiers!",
+    "valid-types": [
+      "string"
+    ],
+    "valid-identifiers": [
+      "normal",
+      "none"
+    ]
+  },
   "cursor": {
     "inherited": true,
     "initial": "auto",