Support BigInt values

[eemeli]

The JavaScript number is a 64-bit floating-point value, which means that for large values (above Number.MIN_SAFE_INTEGER, 2**53 - 1) it is unable to represent integers with complete accuracy. To help with that, BigInt is now a part of the ECMAScript 2020 spec.

This PR adds support for stringifying BigInt values, and if the asBigInt scalar option is set, to parse all integers into BigInts. I also added YAML.scalarOptions as a more user-friendly endpoint for scalar options:

YAML.parse('42')
// 42

YAML.scalarOptions.int.asBigInt = true
const n = YAML.parse('42')
// 42n

YAML.stringify(n)
// '42\n'