[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