Organize build, allow one entrypoint per file

[franzaps]

Adds one entrypoint per file such that import { generatePrivateKey } from 'nostr-tools/keys'; can be done from clients without having to import everything but the kitchen sink. For ESM and CJS, the IIFE version still bundles from index.ts

[paulmillr]

lgtm

[fiatjaf]

I don't know how to fix this conflict because I don't understand the meaning of these package.json fields.

[franzaps]

@fiatjaf I changed it to lib/types (instead of lib) just to have it a bit more organized

Not sure about resolveJsonModule

Allows importing modules with a .json extension, which is a common practice in node projects. This includes generating a type for the import based on the static JSON shape.

Maybe keep it and check that the whole thing still works