Use multiple exports and expose ESM and CJS files

[ibc]

Details

• This PR replaces PR Use multiple exports #38.
• Expose rtp.js/packets and rtp.js/utils paths in exports field in package.json. It works no matter the consumer app uses ESM or CommonJS.
• Still expose a single index based entry point for backwards compatibility.
• Expose ESM and CJS files in exports object instead of transpiling TypeScript code only to CommonJS.
• Use module: NodeNext in tsconfig.json, meaning that TypeScript consumers with moduleResolution "node16", 'NodeNext" or "bundler" can import rtp.js/packets and rtp.js/utils paths, while TypeScript projects with older configuration using moduleResolution "commonjs" can still import rtp.js single entry point.
• Internally, rename all .ts files to .mts to make it clear that those contain TypeScript code in ESM syntax rather than TypeScript code in CJS syntax.

Before

Using ESM:

import { packets, utils } from 'rtp.js';

const rtpPacket = new packets.RtpPacket();
const stringLen = utils.getStringByteLength('œæ€');

Using CJS:

const { packets, utils } = require('rtp.js');

const rtpPacket = new packets.RtpPacket();
const stringLen = utils.getStringByteLength('œæ€');

After

Using ESM:

import { RtpPacket } from 'rtp.js/packets';
import { getStringByteLength } from 'rtp.js/utils';

const rtpPacket = new RtpPacket();
const stringLen = getStringByteLength('œæ€');

Using CJS:

const { RtpPacket } = require('rtp.js/packets');
const { getStringByteLength } = require('rtp.js/utils');

const rtpPacket = new RtpPacket();
const stringLen = getStringByteLength('œæ€');

Note: single entry point still possible

Single entry point ("main" or "module" entries in package.json) is also possible for backwards compatibility:

// ESM
import { packets, utils } from 'rtp.js';

// CJS
const { packets, utils } = require('rtp.js'=;