add support for flutter using dart generator

[humblerookie]

Description

We use flutter a lot in my company and have been using arkana for our ios projects, so thought of adding support for flutter.

This builds on an infra similar to @husseinala's https://github.com/rogerluan/arkana/pull/53.

Arkana Arguments

The --lang operator now supports dart.

Arkana YAML config

The existing result_path allows you to specify a subfolder path/location in the dart project.

Generated Assets

{namespace}.dart

{namespace}_environment.dart

{namespace}_tests.dart

Footnotes

The import path in tests file is currently relative although absolute/package imports are preferred. However, package imports would require us to

• read the dart project's pubspec for namespace
• or add a new parameter both of which I find to be overkill.

I'd appreciate any recommendations and love to hear your thoughts on this.