LibJS: Add side-effect-free version of Value::to_string()

There are now two API's on Value:

- Value::to_string(Interpreter&) -- may throw.
- Value::to_string_without_side_effects() -- will never throw.

These are some pretty big sweeping changes, so it's possible that I did
some part the wrong way. We'll work it out as we go. :^)

Fixes #2123.

Libraries/LibJS/Tests/array-spread.js

@@ -30,6 +30,7 @@ try {
         message: "1 is not iterable",
     });
 
+
     assertThrowsError(() => {
         [...{}];
     }, {