[Chore] Update credo config to support check private vs public functions order

[byhbt]

What happened 👀

• Update the .credo.exs to support the convention Prefer placing all private functions at the bottom of the module. on Compass:

Link: https://nimblehq.co/compass/development/code-conventions/elixir/#module

Insight 📝

We need to ignore the check for ignore: ~w/callback_impl/a because the generated template Phoenix does not consistently follow this layout, for example:

• https://github.com/phoenixframework/phoenix/blob/master/installer/templates/phx_web/telemetry.ex#L9
• https://github.com/phoenixframework/phoenix/blob/master/lib/phoenix/socket/serializers/v2_json_serializer.ex#L37

Therefore, we ignored the callback_impl to keep the .credo.exs follow the existing output of the generated Phoenix structure, just support for the public and private method function order verification.

Proof Of Work 📹

The tests should pass and the .credo.exs output should contain public_fun private_fun

[andyduong1920]

Thanks @byhbt