LibWeb: Support quotes in content values

This adds initial support for `open-quote`, `close-quote`,
`no-open-quote` and `no-close-quote`. We don't yet track the "nesting
level" so we always use the first pair of quotes from the `quotes`
property.

Userland/Libraries/LibWeb/CSS/Identifiers.json

@@ -101,6 +101,7 @@
   "center",
   "circle",
   "clip",
+  "close-quote",
   "coarse",
   "col-resize",
   "collapse",
@@ -240,7 +241,9 @@
   "ne-resize",
   "nearest",
   "nesw-resize",
+  "no-close-quote",
   "no-drop",
+  "no-open-quote",
   "no-preference",
   "no-repeat",
   "none",
@@ -253,6 +256,7 @@
   "nwse-resize",
   "oblique",
   "opaque",
+  "open-quote",
   "optimizequality",
   "optimizespeed",
   "outset",