LibWeb: Add Storage interface and window.localStorage

This is a naive-but-somewhat-functional initial implementation of
HTML Storage.

Note that there is no persistence yet, everything is in-process only,
and one local Storage object per origin.

Userland/Libraries/LibWeb/HTML/Storage.idl

@@ -0,0 +1,11 @@
+[Exposed=Window]
+interface Storage {
+
+    readonly attribute unsigned long length;
+    DOMString? key(unsigned long index);
+    getter DOMString? getItem(DOMString key);
+    setter undefined setItem(DOMString key, DOMString value);
+    deleter undefined removeItem(DOMString key);
+    undefined clear();
+
+};