Allow importing directly from pundit package

[chrisalley]

Currently we're including too much code in the package itself, including both dist and src directories. The way the node_modules subfolder is structured means that we cannot import from the pundit package directly.

• An index.js should be included in the root of the package
• We shouldn't include uncompiled TypeScript code in the package
• import { Policy, When, PunditProvider } from 'pundit/dist/pundit.mjs' should become import { Policy, When, PunditProvider } from 'pundit'