Minor refactoring of the sync algo

[EQUENOS]

Summary

Added InteractionBotBase.application_commands_iterator property that uses itertools.chain to concatenate 3 iterators.
Some methods are now iterating through .application_commands_iterator instead of application_commands (a set) which is of course more efficient.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pre-commit run --all-files
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[thesadru]

Wouldn't it be better to make the iterator a function instead? I think it's normal to assume that next(object.iterator) will affect the attribute and making the iterator a property could cause some issues with readability.

[EQUENOS]

Good point