Add support for BigInt

[aaronhuggins]

Similar to handling of number conversion to integer, BigInt should be handled. Currently, BigInt falls through all detection to be handled as a CBOR map.

import { decode, encode } from './mod.ts'

const bigint = 101n
const number = 100
const a = new Map<any, any>([
  [bigint, bigint],
  [number, number]
])
const encoded = encode(a)
const decoded = decode(encoded, null, { dictionary: 'map' })

console.log(decoded)
// Output: Map { Map {} => Map {}, 100 => 100 }

This is also a prerequisite for proper round-trip of large integer values between CBOR implementations.

[aaronhuggins]

Finished in commit f4ebdf7177240065cb389d7f2fcdf60c45d7e4c0