feat: add SubcommandDeclarationBuilder interface

[DCodeProg]

This pull request introduces support for registering sub-commands in the command declaration builder, improving the structure and extensibility of command definitions. It adds a new SubCommandDeclaration interface and integrates it into the command builder workflow, allowing for more modular and type-safe sub-command registration.

Sub-command support and integration:

• Added a new SubCommandDeclaration interface in sub_command_declaration.dart, requiring implementers to provide a build method that returns a SubCommandBuilder. This also enforces that sub-commands are Listenable.
• Updated CommandDeclarationBuilder to include a registerSubCommand method, which registers sub-commands by accepting a factory function that must return a SubCommandDeclaration instance. This method validates the type and adds the built sub-command to the builder.
• Imported the new sub_command_declaration.dart in both command_declaration_builder.dart and the public API file api.dart to make the interface available for use and export. [1] [2]