Add next branch to use latest react-admin 3.0.0.beta

[zhouhao27]

I added a branch called next which compatible with react-admin 3.0.0.beta.

[rafalzawadzki]

Hello @zhouhao27 thank you for the PR!
As it points to master, we can't merge it. I created a new next branch and changed your PR to point to it. I also cherry-picked your commit regarding the main field (for yarn) to master.

Regarding your changes:

1. May I ask you to rebase on the current next branch
2. Could you change the react-admin dependency to ^3.0.0? This way we will be able to quickly get out of the beta.