Refactor completions settings

[DannyBen]

This PR removes the production-aware completion setting enable_completions and instead, adds this:

# Generate native runtime completions. Supported values:
# - ~ or false     # disable completions (default)
# - minimal        # generate the runtime completion engine without shell adapters
# - bash or zsh    # generate the runtime engine and one shell adapter
# - bash,zsh       # generate the runtime engine and selected shell adapters
# - full           # generate the runtime engine and all available shell adapters
completions: ~

to allow for a higher flexibility of what is generated.