LibWeb: Extract the InnerHTML IDL mixin

Author: https://github.com/AtkinsSJ
Commit: 200e111af0
Pull-request: https://github.com/SerenityOS/serenity/pull/14716
Reviewed-by: https://github.com/kennethmyhra
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWeb/DOM/InnerHTML.idl

@@ -0,0 +1,7 @@
+// https://w3c.github.io/DOM-Parsing/#the-innerhtml-mixin
+interface mixin InnerHTML {
+    [LegacyNullToEmptyString, CEReactions] attribute DOMString innerHTML;
+};
+
+Element includes InnerHTML;
+ShadowRoot includes InnerHTML;