LibWeb: Introduce the concept of "Enumerated" values

This does _NOT_ correspond to anything in the IDL or ECMAScript spec,
but is a custom extended attribute.
We use it to define the "enumerated" values of an attribute

Userland/Libraries/LibWeb/HTML/Scripting/Fetching.idl

@@ -0,0 +1,7 @@
+// https://html.spec.whatwg.org/multipage/urls-and-fetching.html#cors-settings-attribute
+[MissingValueDefault=, InvalidValueDefault=anonymous]
+enum CORSSettingsAttribute {
+    "anonymous",
+    "",
+    "use-credentials"
+};