Fix JSON number parsing

[0xDyews]

Fix JSON number parsing

The JSON number parser was accepting some invalid JSON number formats because it did not enforce the required structure of a JSON number before converting it with std::stod().

For example, values such as +1, 01, 1., and 1e could be accepted even though they are not valid JSON numbers.

This updates parse_number() to validate:

• the optional leading -
• the integer part and leading zero rules
• the fractional part
• the exponent and its optional sign

Valid JSON numbers continue to be accepted.