feat/add Abstract Entity

src/main/java/org/example/Entity.java

+package org.example;
+
+public abstract class Entity {
+    private int damage;
+    private int pvMax;
+    private int pv;
+    private int endurance;
+
+
+    public void heal(){
+        this.pv += this.pvMax/3;
+    } 
+
+    public boolean isAlive(){
+        return (this.pv > 0);
+    }
+}