extend ability system to support dynamic slot counts, add skill tree validation, new prerequisites API, and extended mod commands

src/main/java/net/fretux/skillengine/skilltree/SkillLogic.java

@@ -33,7 +33,7 @@ public class SkillLogic {
 
         int totalCost = 0;
         for (SkillNode plannedNode : plan) {
-            totalCost += plannedNode.getCost();
+            totalCost += Math.max(0, plannedNode.getCost());
         }
         if (data.getSkillPoints() < totalCost) {
             return Optional.empty();

src/main/java/net/fretux/skillengine/skilltree/SkillTreeValidator.java

@@ -33,6 +33,9 @@ public final class SkillTreeValidator {
         if (!isPngTexture(node.getIcons())) {
             issues.add(node.getId() + " has invalid icon path " + node.getIcons() + " (expected a .png texture)");
         }
+        if (node.getCost() < 0) {
+            issues.add(node.getId() + " has negative cost " + node.getCost());
+        }
         for (ResourceLocation parent : node.getLinks()) {
             if (SkillNodeRegistry.get(parent) == null) {
                 issues.add(node.getId() + " links to missing skill node " + parent);
@@ -83,6 +86,9 @@ public final class SkillTreeValidator {
         if (!isPngTexture(ability.getIcon())) {
             issues.add(ability.getId() + " has invalid icon path " + ability.getIcon() + " (expected a .png texture)");
         }
+        if (ability.getCooldown() < 0) {
+            issues.add(ability.getId() + " has negative cooldown " + ability.getCooldown());
+        }
         for (ResourceLocation parent : ability.getLinks()) {
             if (SkillNodeRegistry.get(parent) == null && AbilityNodeRegistry.get(parent) == null) {
                 issues.add(ability.getId() + " links to missing node or ability " + parent);