Core: ServiceRegistry, GetOrCreate<T> and Remove(instance)

[Eideren]

PR Details

Adds two new methods to the service registry to help manage singleton-like objects and improve safety when un-registering services.

This is the shortest form to get and create a service if it doesn't exist:

if (Services.GetService<MySingleton>() is { } singleton == false)
    Services.AddService(singleton = new MySingleton(Services));
singleton.DoWhatever();

Now this is fine, but singletons are more often than not used all over the place throughout a codebase, meaning that users will either have to copy paste the above, or create an extension method.
This pr would reduce the above into the following:

var singleton = Services.GetOrCreate<MySingleton>();
singleton.DoWhatever();

It is also easier to maintain as users now have only one entry point to their singleton's construction mechanism, see this implementation for the singleton above:

public class MySingleton : IService
{
    public void DoWhatever() { }

    public static IService NewInstance(IServiceRegistry r) => new MySingleton();
}

There are a couple of cases like this one in the engine's source.

The remove is there to improve safety when un-registering them, the following usage would be problematic without the instance safeguard.

public class MySingleton : StartupScript
{
    public override void Start()
    {
        if (Services.GetService<MySingleton>() is not null)
        {
            Entity.Components.Remove(this);
            return;
        }

        Services.AddService(this);
        base.Start();
    }

    public override void Cancel()
    {
        base.Cancel();
        Services.RemoveService(this);
    }
}

And they stay threadsafe, like the rest of this type.

Related Issue

None.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.