List of breaking changes in 2.0.0

[DannyBen]

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

• Rename Settings.enable_bash3_bouncer to Settings.enable_bash_version_bouncer since we are rejecting bash < 4.2 now.
• 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).
• Remove command.completions (use arg.completions instead).
• 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.