Shared class for "Target creature deals damage to itself equal to its power"

Spotted while doing other things, but there's several card-specific implementations for this;

> Target creature deals damage to itself equal to its power

We can use a shared class for this.

Cards in scope:

```
i/InnerStruggle.java
j/JusticeStrike.java
k/KikuNightsFlower.java
k/KikusShadow.java
r/Repentance.java
w/WrackWithMadness.java
```

Start by reading the implementations in i/InnerStruggle.java, j/JusticeStrike.java, k/KikuNightsFlower.java, k/KikusShadow.java, r/Repentance.java, and w/WrackWithMadness.java, then compare how they implement the stated damage effect. The work is done when these card-specific implementations use one shared class while preserving the behavior described in the issue.

Written by the indexing model from the issue text.