[Feature Request] Add an option to retain fly mode after chaning gamemode

[TetraTheta]

Feature description

I want EssentialsX to retain fly mode after changing game modes.

1. Set your game mode to Survival
2. Enable fly mode to yourself
3. Change your game mode to Creative
4. Change your game mode back to Survival
5. Check your fly mode state
   1. [Expected] Fly mode should be enabled
   2. [Actual] Fly mode is automatically disabled

How the feature is useful

I know this would be niche case, and this might be duplicated with #4325.

I feel uncomfortable whenever I switch my game mode to Creative and back to Survival, and my fly mode is automatically disabled so I have to type /fly again and again.

It would be nice to make user to decide whether disable fly mode automatically or not when game mode is changed, via permission or config option. If fly mode is enabled when changing game mode to Survival, just notifying user that fly mode is enabled would be sufficient to tell him that he can fly.

I don't see any option or permission for this currently.