[Buff] Add auto-decreasing buffs that decrease with time

Prepare the engine

• Add a plugin.preTick(delta: number) to the LudiekPlugin with an empty default.
• Update engine.preTick(delta: number) to call all registered plugins preTick(delta) after calculating bonuses.

Implement the behaviour

• Add an optional boolean autoDecrease to the BuffDefinition to toggle this behaviour (default false)
• In the BuffPlugin, implement preTick() to decrease all autoDecreasing buffs by delta

Update the documentation

• Currently the BuffDemonstration.svelte manually ticks down each buff, but now we can just call engine.preTick and it will do it for us!

Add tests

• Add tests that show that buffs with this behaviour go down on preTick, and another to show that buffs without don't