fix: refactor to latest asdf best practices

[jthegedus]

Description

• [x] add Bats testing
• [x] add banned_commands.bats from asdf core to enforce POSIX and cross platform functionality as best as possible
• [x] refactor lib/* dir
• [ ] write tests for utils/helpers

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Usage examples

How Has This Been Tested?

Checklist:

• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.