Fix types for package.json exports (#542, #543, #545)

[fruchtose-stripe]

Summary & motivation

Two fixes here:

1. Types for ES modules need to be specified on a per-module basis, as suggested in Types that are found, but not exported in package.json, should still be usable microsoft/TypeScript#52363 (comment)
2. A separate path reference fix for the pure module

Testing & documentation

I tested this against packages which were compiled in webpack/vite with "moduleResolution": "node16" and "moduleResolution": "bundler" in tsconfig.json. I confirmed this fix works fore TypeScript 4+.