Added MonoSingleton Type

[wqyeo]

Same functionality as Singleton, but you can choose to apply a DontDestroyOnLoad on it via the inspector.

However, it inherits from MonoBehaviour unlike the original Singleton. This is due to it using Awake call to apply a DontDestroyOnLoad on the instance if need.
The child class will need to override an OnAwake function. (simulates an Awake call)

I decided to keep the original Singleton, and made this into MonoSingleton due to the above stated differences.

P.S: Also, should we make the original Singleton class an abstract?

[Deadcows]

Well actually existing Singleton<> is for MonoBehaviours but simply for caching and global access.
So Singleton.Instance.Play(...); will access and cache AudioManager on scene.
It's possible to easily add Instantiation functionality along with Don't destroy on load there.
I like the idea that MB itself is not a singleton, but outer scripts may access it statically :) But it was made for specific case and I didn't used it for a few years now, so...

Personally I don't like base MB classes, but I got your point 👍🏻