Create empty-import.d.ts and reference it in submodule imports

[ifiokjr]

• Fixes Typings not being told in package.json #1111

A full reproduction of the error is available below

https://github.com/davideast/rxfire-samples/blob/f4b0b6b7959b7fb52b04fb7a8041edb17b92a872/src/preact/index.tsx#L20-L23

// NOTE: The lazy load for Firestore does not work with TypeScipt
// w/out hacking the node_modules/firebase/firestore/package.json file
// to use "typings": "../index.d.ts".
const firestore$ = from(import('firebase/firestore'));

[Feiyang1]

Awesome, LGTM. Thank you very much. Can you please close this one and create a new PR on top of the HEAD? We had a hiccup in CI build, so the build fails for this PR.

[ifiokjr]

@Feiyang1 I've just rebased from the latest master.

Also I've added another typings fix for rxfire/database. Currently it's not being referenced in the package.json.

[Feiyang1]

LGTM

[ifiokjr]

@Feiyang1 thanks for approving! 👍
@davideast Any chance this could be merged soon. It'll save me from the ugly post-install bash script I'm currently using to bypass the issue.