LibWeb: Add support for HTMLOrSVGElement.dataset

Author: https://github.com/Lubrsi
Commit: f6b24a72ee
Pull-request: https://github.com/SerenityOS/serenity/pull/10219

Userland/Libraries/LibWeb/HTML/DOMStringMap.idl

@@ -0,0 +1,8 @@
+[Exposed=Window, LegacyOverrideBuiltIns]
+interface DOMStringMap {
+    getter DOMString (DOMString name);
+
+    // FIXME: Extended attributes are currently not applied to special operations.
+    [CEReactions] setter undefined (DOMString name, DOMString value);
+    [CEReactions] deleter undefined (DOMString name);
+};