https://github.com/marmelab/react-admin/blob/83dc1d61f0d7efaafebb57885d6b55f78509b581/packages/ra-ui-materialui/src/form/FormTabHeader.tsx has an import from `react-is`, but it is not declared in `package.json`

This was causing errors in my rollup build. I cannot be sure this is the right fix - for my project, I had to add this in my `.yarmrc.yml`:

```
packageExtensions:
  "ra-ui-materialui@*":
    dependencies:
      "react-is": "*"
```
Here, `peerDependencies` doesn't work, but for `package.json` I think it makes sense to put it there.

…ckage