Expose core.init outside of frb_generated.dart

We need to call `core.init()` to initialize payjoin-flutter in isolates. We can get this from `package:payjoin_flutter/src/generated/frb_generated.dart` but that gives a warning:

```
Import of a library in the 'lib/src' directory of another package.
Try importing a public library that exports this library, or removing the import
```