feat(ESModules): importing for esm modules

[duncte123]

Please describe the changes this PR makes and why it should be merged:
This PR fixes #3992 by adding special exports for esm modules.
The exports in the package.json have been inspired by eris with modifications to work with discord.js

I've built a script that automatically can build the discord.mjs file if needed, the script is as follows:

const { writeFileSync } = require('fs');
const Discord = require('discord.js');
const discordExports = Object.keys(Discord);

const template = `import Discord from "../src/index.js";

export default Discord;

export const {
  ${discordExports.join(",\n  ")}
} = Discord;
`;

writeFileSync("./esm/discord.mjs", template);

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[ghost]

Node.js fully provides support for ECMAScript modules as they provide limited interoperability between them and the existing module format, namely, CommonJS.

Due to the increasing popularity of ECMAScript modules, it is utterly reasonable to allow support for ECMAScript modules. One drawback of the change is that it blocks 'subpath imports' if paths are not represented in the exports field. This, however, can be utilized using the subpath, which is already defined in the exports field, ./esm.

import submodule from 'discord.js/esm';

[iCrawl]

I'd argue this is more feat (feature), than a fix here.

I assume this is absolutely needed to get modules to work in Node.js?
Pardon my ignorance on that topic.

[duncte123]

I'd argue this is more feat (feature), than a fix here.

I assume this is absolutely needed to get modules to work in Node.js?
Pardon my ignorance on that topic.

Yes this change is required to get modules working in node.js that is why I labeled it as a fix, documentation can be found on this page https://nodejs.org/api/esm.html

A workaround is achievable by using the esm package but that is unnecessary bloat added to a project.