LibJS: Implement correct object property ordering

This commit introduces a way to get an object's own properties in the
correct order. The "correct order" for JS object properties is first all
array-like index properties (numeric keys) sorted by insertion order,
followed by all string properties sorted by insertion order.

Objects also now print correctly in the repl! Before this commit:

courage ~/js-tests $ js
> ({ foo: 1, bar: 2, baz: 3 })
{ bar: 2, foo: 1, baz: 3 }

After:

courage ~/js-tests $ js
> ({ foo: 1, bar: 2, baz: 3 })
{ foo: 1, bar: 2, baz: 3 }

Libraries/LibJS/Runtime/Cell.cpp

@@ -56,6 +56,11 @@ Interpreter& Cell::interpreter()
     return heap().interpreter();
 }
 
+Interpreter& Cell::interpreter() const
+{
+    return heap().interpreter();
+}
+
 const LogStream& operator<<(const LogStream& stream, const Cell* cell)
 {
     if (!cell)