[web] Move createAsyncMigrate to lib

Summary:
https://linear.app/comm/issue/ENG-3997/support-async-migrations-on-native
We want to support async migrations on native, as we do on the web. This diffs moves the createAsyncMigrate function to lib, and extracts web-specific code.

Test Plan:
(tested on firefox)

1. Remove firefox from DB_SUPPORTED_BROWSERS, change stateVersion on web from 2 to 1, clear all data for localhost in firefox
2. Open the web app, check if indexedDB is empty, create a dummy draft
3. Add firefox to supported browsers, bump stateVersion to 2
4. Open the app, check if the draft is still there, check if indexedDB has data
5. Refresh the app check if the draft is still there

Reviewers: kamil, tomek, ashoat

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8352