Implement global undefined property

[HalidOdat]

ECMASCript feature

undefined is not a keyword, but a global property.

We can add this by adding a new module like NaN https://github.com/boa-dev/boa/tree/master/boa/src/builtins/nan and then adding it to the builtin init function in builtins/mod.rs the value it should contain should be Value::undefined()

More information
- ECMAScript reference
- MDN

[AnirudhKonduru]

@HalidOdat , I could use some help with testing this. Doing it like NaN is tested won't do much, as

let scenario = r#"
        undeclared_identifier; //or undefined;
        "#;

will always evaluate to undefined, even when we don't add undefined as a global property.

[Razican]

@HalidOdat , I could use some help with testing this. Doing it like NaN is tested won't do much, as

let scenario = r#"
        undeclared_identifier; //or undefined;
        "#;

will always evaluate to undefined, even when we don't add undefined as a global property.

Yes, this is a bug we have, where we don't throw a ReferenceError exception. This should be solved once #488 gets merged, though.