Change "priority" on System to be a static member

[mattjennings]

Context

This would make it much easier to specify a system to run before/after another system.

class SomeSystem extends ex.System {
    static priority = ex.MotionSystem.priority - 1
}

[eonarheim]

I support this, you thinking like this?

This would definitely be a breaking change for anyone that's implemented a custom system

[mattjennings]

Yup, exactly.

It appears you can actually have both an instance and static member of the same name (TIL), if we wanted to avoid the breaking change.

[github-actions[bot]]

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