`atmos terraform shell` should be a top level command and not a subcommand

[Gowiem]

Describe the Feature

atmos terraform shell should be a top level command parsed by Cobra and not a pass-through so it can have proper help documentation.

Expected Behavior

When I run atmos terraform shell --help, I get help docs for the shell command.

Use Case

Allow the shell command to better documented and useful to atmos users.

Describe Ideal Solution

Similar to atmos terraform generate, shell will be documentable via cobra docs and allow for better usage from atmos users.

Alternatives Considered

N/A

Additional Context

I would like to start contributing, so I can implement this if we agree to the approach, but figured I should bring it up for discussion before throwing up a PR.