Task DSL for arg and positional_arg should warn about naming collisions

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This will put you in a world of confusion:

```crystal
class MyTask < LuckyCli::Task
arg :summary, "", optional: true
positional_arg :name, ""

def call(io : IO = STDOUT)
# do stuff
end
end
```

Macro-generated args can easily conflict with the in-use internal namespace in the task infrastructure. When you run `lucky -h` with that task in your tasks directory, you'll get an opaque error because calling `.name` on the class isn't doing what lucky_cli thought it would do. You'll get:

`Unhandled exception: name is required, but no value was passed. (Exception)`

I wonder if it makes sense to:

- namespace the LuckyCli::Task infrastructure, for example instead of a `summary` method being generated from the `summary` macro, maybe it generates a `_task_summary` method.
- encapsulate the declared arguments within an Args object of some sort. Within a task, a `arg :summary` would be accessed via `args.summary` instead of just `summary`.
- simply warn the developer that they're about to shoot themselves in the foot by declaring an arg with a name of: name, summary, help_message, call, etc

Start with the macro-generated implementations of `arg` and `positional_arg`, then reproduce the collision by running `lucky -h` with the task shown in the issue. Done means the conflicting names are identified and a clear warning is emitted or an agreed encapsulation prevents the opaque error.

Written by the indexing model from the issue text.