feat(Esm): use gen-esm-wrapper instead of manually making the file

[vladfrangu]

Please describe the changes this PR makes and why it should be merged:

One less file to maintain! 🎉

Status and versioning classification:

N/A?

[kyranet]

@vladfrangu this needs a rebase.

[zt64]

I don't really understand this too much, but I decided to switch to TypeScript and spent over an hour trying to find why it wouldn't work. Eventually I saw that it had no mjs file and it led me to this PR, then manually running gen-esm-wrapper ./src/index.js ./src/index.mjs fixed it. I don't know if this is supposed to automatically run sometime or I'm just misunderstanding something

[iShibi]

It runs automatically when the library gets packaged and published to npm. So, you wouldn't have to do this if you install from npm using npm i discord.js@dev

[monbrey]

I decided to switch to TypeScript and spent over an hour trying to find why it wouldn't work. Eventually I saw that it had no mjs file

TypeScript doesn't use the .mjs file, it's only for using ES6 syntax in regular JavaScript