Lexer: Evaluate token values

After the first pass-through and token generation, we need to evaluate for each `Token` its runtime value.

e.g.

```go
Token{T: T.NumericLiteral, Lexeme: "0xAB"}
```

Converted to

```go
Token{T: T.NumericLiteral, Lexeme: "0xAB", Value: 171}
```

https://262.ecma-international.org/#sec-static-semantics-mv