LibWeb: Add HistoryHandlingBehavior::Push

Also add FIXMEs about removing no-longer-used enum values.

Userland/Libraries/LibWeb/HTML/HistoryHandlingBehavior.h

@@ -10,9 +10,10 @@ namespace Web::HTML {
 
 // https://html.spec.whatwg.org/multipage/browsing-the-web.html#history-handling-behavior
 enum class HistoryHandlingBehavior {
-    Default,
-    EntryUpdate,
-    Reload,
+    Default,     // FIXME: This is no longer part of the spec. Remove.
+    EntryUpdate, // FIXME: This is no longer part of the spec. Remove.
+    Reload,      // FIXME: This is no longer part of the spec. Remove.
+    Push,
     Replace,
 };