Allow usage of `proprietary` license in composer.json

With version `@flarum/cli/2.0.0-beta.17 darwin-arm64 node-v20.6.0` it's currently not possible to use any other license than the official SPDX licenses, which in principle makes sense. A workaround for using the CLI is to temporarily use a different, valid SPDX license and change it back after.

When trying to use the cli anyways with an invalid SPDX license, such as `proprietary`, the following error is thrown:

```
Error occurred, and could not complete:
Cannot find module 'spdx-license-list/licenses/proprietary'
```

Therefore, I would propose that `proprietary` is added as an exception and made available to developers working on private extensions. I'm just throwing this idea out there, I have no idea if this can even feasibly be done in this repo.

Thoughts?