LibJS: Loosen type system

This commits makes effort towards tolerating some of javascript's quirks
when it comes to its type system, note that the interpreter's way of
handling type coercion is still not mature at all, for example, we still
have to implement NaN instead of just crashing when trying to parse a
string and failing.

Libraries/LibJS/Cell.cpp

@@ -40,7 +40,7 @@ void Cell::Visitor::visit(Value value)
         visit(value.as_cell());
 }
 
-Heap& Cell::heap()
+Heap& Cell::heap() const
 {
     return HeapBlock::from_cell(this)->heap();
 }