Parse `NaN` and `Infinity / -Infinity`

[LuoZijun]

Keywords:

• NaN / +NaN / -NaN --> std::f64::NAN
• Infinity / +Infinity --> std::f64::INFINITY
• -Infinity --> std::f64::NEG_INFINITY

These keywords need to be parsed into Literal, not an Identifier .

[cmtt]

NaN and Infinity are properties of the global object (window in a browser environment), e.g. acessible as global.NaN or global.Inifnity.

Thus, they are not parsed as special literals, see also the exhaustive list of literals in the ECMAScript 2015 specification.