chore: add "./package.json" to exports

[shigma]

Add ./package.json to exports for package locating.

When bundling a node library, one may need to copy the dist/entry directory into output directory (because it is not directly referenced). It is useful to have a package.json export when one uses build script like this:

require.resolve('tinypool/package.json')

It is also common practice for popular packages such as vite.

https://github.com/vitejs/vite/blob/1025bb6d8f21c0cb9fe72405d42e0f91bb3f1d8e/packages/vite/package.json#L43

[AriPerkkio]

Looks good, thanks!