Fix update of userspace config when running qmk userspace-add interactively

[zvecr]

Description

Currently, if you run qmk userspace-add with no arguments, it will prompt the user for various inputs. In this case, the keyboard/keymap arguments are not known by userspace_add.

The userspace config is updated within the new_keymap call without the potential cli.args.env value. Another attempt to save then happens as part of userspace_add with its own view of the current state, but as the keyboard/keymap args are empty the entry is then missing.

This PR refactors the cli logic into something that is callable with args, instead of the population of a cli object. It also updates the logic so that a save only happens once, and with the correct keyboard/keymap arguments regardless of if they originated from command line or prompt.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).