Refine `number` definition

[tjanc]

The specification doesn't specify what an MSON number is, apart from it being a primitive type.
Drafter currently uses the ECMA-404 (JSON) number definition quoted below. I propose anchoring it into the MSON spec.

A number is a sequence of decimal digits with no superfluous
leading zero. It may have a preceding minus sign (U+002D).
It may have a fractional part prefixed by a decimal point (U+002E).
It may have an exponent, prefixed by e (U+0065) or E (U+0045) and
optionally + (U+002B) or – (U+002D) . The digits are the code points
U+0030 through U+0039.