Add option to prefer decoding as Map

[ninevra]

This PR adds an option, preferMap, which when set causes all MessagePack maps to be decoded to javascript Maps.

The preexisting default behavior is to decode as Map if non-string keys exist and as a plain object otherwise. This means that decode(encode(new Map()) is {}, i.e., Maps do not round-trip reliably. It also prevents the user from reliably determining the order of entries in the encoded MessagePack map. (It's not clear to me if MessagePack intends users to care about map order, but the spec doesn't appear to prohibit it.) An example:

const {decode, encode} = require('msgpack5')();
const map = new Map()
  .set('b', 1)
  .set('1', 2)
  .set('01', 3)
  .set('0', 4)
  .set('a', 5);
const buffer = encode(map); // <Buffer 85 a1 62 01 a1 31 02 a2 30 31 03 a1 30 04 a1 61 05>
const object = decode(map);
const keys = Object.keys(object); // ['0', '1', 'b', '01', 'a']

The Map's order is reflected by the encoded data but replaced by js object traversal order in the decoded data.

With preferMap: true, decode(encode(new Map()) is new Map(), and the example above decodes to a Map with key ordering 'b', '1', '01', '0', 'a'.

[mcollina]

lgtm