FR: Support module import in the "firebase" package for Nodejs 13/14

Submodule resolution algorithm for module import has changed since Nodejs 13 and our current way of defining submodules using nested `package.json`s doesn't work anymore, see https://github.com/nodejs/node/issues/33410

We should use the `"exports"` field in the root package.json instead:
```
{
"exports": {
"./app": "./app/dist/index.cjs.js"
}
}
```

Note that this only affects importing firebase in Nodejs.