LibWeb: Flesh out prepare_script and execute_script

This fills in a bunch of the FIXMEs that was in prepare_script.

execute_script is almost finished, it's just missing the module side.

As an aside, let's not assert when inserting a script element with
innerHTML.

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -30,6 +30,7 @@ interface Document : Node {
     readonly attribute Element? documentElement;
     attribute HTMLElement? body;
     readonly attribute HTMLHeadElement? head;
+    readonly attribute HTMLScriptElement? currentScript;
 
     readonly attribute DOMString readyState;