Small build improvement

[EpsilonD3lta]

This is an insignificant improvement, but code starting on line https://github.com/Baste-RainGames/PlayerLoopInterface/blob/141abb7e6d4277107a7d760e71d645fbaeeb5d4f/Runtime/PlayerLoopInterface.cs#L32

    [RuntimeInitializeOnLoadMethod]
    private static void Initialize() {
        // Systems are not automatically removed from the PlayerLoop, so we need to clean up the ones that have been added in play mode, as they'd otherwise
        // keep running when outside play mode, and in the next play mode if we don't have assembly reload turned on.

        Application.quitting += ClearInsertedSystems;
    }

    private static void ClearInsertedSystems ()
    {
        foreach (var playerLoopSystem in insertedSystems)
            TryRemoveSystem(playerLoopSystem.type);

        insertedSystems.Clear();

        Application.quitting -= ClearInsertedSystems;
    }

could be probably wrapped in #IF_UNITY_EDITOR, as it's not necessary in build.

Even smaller thing: consider using [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)], probably does not change anything (I tested edge case when I called EditorApplication.ExitPlaymode(); from other [RuntimeInitializeOnLoadMehthod] method and this still got executed), but it exists for purposes like this.

Btw great package, thank you for making it available for everyone.

[Baste-RainGames]

You're right on both counts!

Clearing the systems in builds is pretty bad, since it (ever so slightly) increases the time it takes to close the game. The ordering of the [RuntimeInitializeOnLoad] call isn't super important since it just hooks up to a message so it doesn't interact with the ordering of other systems, but SubSystemRegistration is more appropriate, yes.

Feel free to send a PR that includes the changes. If not, I'll get around to it eventually.

[EpsilonD3lta]

I created a PR #4. I did not test the code in a build yet. It's a simple change, but I will be making builds in few days so if it doesn't work I'll be back.