Proposal: Support parsing JSON number as RawNumber type

In order to parse JSON numbers with full precision and improve parsing performance, we propose adding a new `RawNumber` type for representing parsed JSON number values.

The design of this feature includes the following steps:

1. Add a new `kRawNumber` enum to represent JSON number values, and add a `RawNumber`class to distinguish it from `stringview` type.
2. Provide support for construct, modifying, copying, and comparing `RawNumber` values. Especially, validate the number in construct and set API, because the number is from the user and maybe invalid numbers
3. Use `IsRawNumber` to check for the presence of a `RawNumber`, while `IsNumber` continues to check for numeric types.

**Implementation branch:** #60
Status: To be reviewed.