NaN is lexed as identifier, not as a number

[hello2dj]

Describe the bug
When lexing NaN, we don't get a number, we get an identifier.

To Reproduce
Run this JavaScript with cargo run -- -t {file}.js:

let a = NaN;

Expected behavior
The token for NaN should be :

Token {
    kind: NumericLiteral(
        Rational(NaN),
    ),
    pos: Position {
        column_number: 9,
        line_number: 1,
    },
},

Instead, we are getting:

Token {
    kind: Identifier(
        "NaN",
    ),
    pos: Position {
        column_number: 9,
        line_number: 1,
    },
},

This is controlled in https://github.com/jasonwilliams/boa/blob/master/boa/src/syntax/lexer/mod.rs#L591-L602. Appart from checking for true, false and null, we should also check for NaN and assign the correct rational, containing f64::NaN.

[Razican]

There are two issues related to this. First, #359 makes any call to typeof panic. But since that's being tracked there, this should track the fact that NaN is not lexed as a number, it's lexed as an identifier.

[Razican]

I updated the description, so that it's easy to reproduce and fix :)