LibWeb: Add IDL enum for fetch priority attribute

Userland/Libraries/LibWeb/HTML/AttributeNames.h

@@ -67,6 +67,7 @@ namespace AttributeNames {
     __ENUMERATE_HTML_ATTRIBUTE(enctype)                    \
     __ENUMERATE_HTML_ATTRIBUTE(event)                      \
     __ENUMERATE_HTML_ATTRIBUTE(face)                       \
+    __ENUMERATE_HTML_ATTRIBUTE(fetchpriority)              \
     __ENUMERATE_HTML_ATTRIBUTE(for_)                       \
     __ENUMERATE_HTML_ATTRIBUTE(form)                       \
     __ENUMERATE_HTML_ATTRIBUTE(formaction)                 \

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

@@ -11,3 +11,11 @@ enum LazyLoadingAttribute {
     "lazy",
     "eager"
 };
+
+// https://html.spec.whatwg.org/multipage/urls-and-fetching.html#fetch-priority-attributes
+[MissingValueDefault=auto, InvalidValueDefault=auto]
+enum FetchPriorityAttribute {
+    "high",
+    "low",
+    "auto"
+};