LibJS: Fix conditional expression precedence

This fixes the following from parsing incorrectly due to the comma
that occurs after the conditional:

  let o = {
    foo: true ? 1 : 2,
    bar: 'baz',
  };

Libraries/LibJS/Tests/ternary-basic.js

@@ -6,6 +6,8 @@ try {
     assert(x === 1 ? true : false);
     assert((x ? x : 0) === x);
     assert(1 < 2 ? (true) : (false));
+    assert((0 ? 1 : 1 ? 10 : 20) === 10);
+    assert((0 ? 1 ? 1 : 10 : 20) === 20);
 
     var o = {};
     o.f = true;