XS: Numeric literals not parsed according to specification

[tevador]

According to the ECMAScript 6 specification, the exponent part of a DecimalLiteral consists of the ExponentIndicator (letter e or E) followed by a SignedInteger.

However, it seems that the XS interpreter doesn't require any number to follow the ExponentIndicator:

./xst -e "print(884e)"
NaN

This should result in a SyntaxError and not NaN, because the exponent part is incomplete.

[tevador]

Fixed here: tevador@8222b51