Fix: export types correctly

[christian98]

The "exports" field of the package.json was misused for the type-declarations. Also, the type-declarations of secondary entrypoint (server.ts) of the platform packages was not generated.

Docs:

• https://2ality.com/2021/06/typescript-esm-nodejs.html#mapping-a-directory-to-a-file
• https://dev.to/binjospookie/exports-in-package-json-1fl2

[jessarcher]

I've just rebased this with the latest changes from master to confirm everything is working and to get the new CI checks to run.

I've confirmed it's an improvement and fixes the issue with the server type exports (evidenced by needing to remove two @ts-expect-error directives in the Vue 3 playground 🎉)

Thanks!

[jessarcher]

It has been brought to my attention that the package.json exports is a valid way to provide type declarations, and it has priority over typesVersions.

microsoft/TypeScript#50890

I'm going to submit an alternative version of this PR that should fix the server export issues.

[jessarcher]

It appears I was too hasty in closing this PR. The exports.*.types feature is only for projects with moduleResolution set to node16 or nodenext, and while it would be nice to move towards that, I don't want it to delay this fix any longer.

[jessarcher]

I've restored the exports.*.types fields, but fixed the file extension on them, and also moved them to the top of their respective sections per https://nodejs.org/api/packages.html#community-conditions-definitions