List of future breaking changes

[DannyBen]

This is a live issue that lists future breaking changes that we might want to implement when releasing the next major version.

1. Rename Settings.enable_bash3_bouncer to Settings.enable_bash_version_bouncer since we are rejecting bash < 4.2 now.
2. Consider renaming run() to {{ name }}() or something else, to free up this multi-use, potentially conflicting function name (cc Rename run function to main_run #706).
3. Remove command.completions (use arg.completions instead).
4. Refactor completions without completely. Instead, we could use the bashly-native data model and generate a private __complete function, inspired by Go's Cobra. This would allow obeying uniques, conflicts, and anything that bashly supports.