Allow defining a stage for system's execution

[skyne98]

Like in bevy here.

Some systems might want to run before, or after, all other normal systems. Currently, it's impossible to do, unless you make this system manually require the execution of all those systems.

Stages to support at first:

• PreSetup, Setup
• First, PreUpdate, Update, PostUpdate, Last

By default, systems are added to the Update stage and are executed in the order of being added, unless manually requiring to execute, breaking the order.
Also, requiring execution of a system from a different stage should result in an error, as it moves the system execution out of the desired stage and might break things.