LibJS: Implement bytecode generation for For-In/Of statements

This also implements the rather interesting behaviour that #12772 relies
on, so this fixes that bug in BC mode (the AST interp remains affected).

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -31,6 +31,7 @@
     O(GetById)                       \
     O(GetByValue)                    \
     O(GetIterator)                   \
+    O(GetObjectPropertyIterator)     \
     O(GetVariable)                   \
     O(GreaterThan)                   \
     O(GreaterThanEquals)             \