LibJS+LibWeb+WebContent: Port JS::PropertyKey to UTF-16

This has quite a lot of fall out. But the majority of it is just type or
UDL substitution, where the changes just fall through to other function
calls.

By changing property key storage to UTF-16, the main affected areas are:
* NativeFunction names must now be UTF-16
* Bytecode identifiers must now be UTF-16
* Module/binding names must now be UTF-16

Libraries/LibJS/Bytecode/Generator.h

@@ -212,7 +212,12 @@ public:
         return m_regex_table->insert(move(regex));
     }
 
-    IdentifierTableIndex intern_identifier(FlyString string)
+    IdentifierTableIndex intern_identifier(FlyString const& string)
+    {
+        return intern_identifier(Utf16FlyString::from_utf8(string));
+    }
+
+    IdentifierTableIndex intern_identifier(Utf16FlyString string)
     {
         return m_identifier_table->insert(move(string));
     }