[WIP] refactor: moved style and package_parameter_enabled to under assets.outputs.

[wasabeef]

What does this change?

before

flutter_gen:
  # ...
  assets:
    enabled: true
    package_parameter_enabled: true
    style: snake-case
    outputs:
      class_name: MyAssets

v5.0.0 or later.

  # ...
  assets:
    enabled: true
    outputs:
      class_name: MyAssets
      package_parameter_enabled: true
      style: snake-case