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

## 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.