using a [babel plugin](https://gist.github.com/pengx17/49e24ae8a5a609bdaff122ee8c679d1c) to transform all var(--affine-xxx) to cssVar
Some issues:
- tried ast-grep but it seems to be not easy to add imports conditionally
- current work does not work well with ts with types because babel will strip them out
If network offline or API error happens, the `session` returned by the `useSession` hook will be null, so we can't assume it is not null.
There should be following changes:
1. create a page in ErrorBoundary to let the user refetch the session.
2. The `SessionProvider` stop to pull the new session once the session is null, we need to figure out a way to pull the new session when the network is back or the user click the refetch button.
Peng Xiao