fix NaN is lexed as identifier, not as a number

[attila-lin]

This Pull Request fixes/closes #393.

It changes the following:

• make NaN a number
• add test for it

[Razican]

This is perfect, thanks for the contribution!

[jasonwilliams]

yep, looks good

[HalidOdat]

Looks good! Thanks for contributing!

There is an edge case we need to support in the future which is

var NaN = 4;

which requires changes to the parser too.
This also applies to undefined.

[simonbuchan]

This is fine as a hack, but NaN is actually specified as a global variable, like undefined, not a keyword, like null. (JavaScript is terrible.)
https://www.ecma-international.org/ecma-262/10.0/index.html#sec-global-object

[Razican]

This is fine as a hack, but NaN is actually specified as a global variable, like undefined, not a keyword, like null. (JavaScript is terrible.)
https://www.ecma-international.org/ecma-262/10.0/index.html#sec-global-object

Yep, we will have to improve this in the future, but first, we need to improve the lexer, the current one is a bit difficult to follow, it became too big.

I opened #421 regarding this.