Flutter IntelliJ Plugin M54.1 Release

[Steve Messick]

We've added support for IntelliJ 2021.1 Beta. 

We also fixed an issue that was preventing platform selection to be made during project creation. That fix applies to all supported IntelliJ versions as well as the beta and canary channels of Android Studio.

Unfortunately, it does not apply to Android Studio 4.1 (stable). To create a project with custom platforms run `flutter create` from the command line. Use the menu path `View > Tool Windows > Terminal` and type the command in the terminal emulator. For more details on using Flutter on the command line, see the getting started guide at https://flutter.dev/docs/get-started/install .