feat: Add CodeXPChallenge data model for CodeXP challenges

Author: ByteAurora

src/main/kotlin/com/github/ilovegamecoding/intellijcodexp/model/CodeXPChallenge.kt

@@ -0,0 +1,17 @@
+package com.github.ilovegamecoding.intellijcodexp.model
+
+data class CodeXPChallenge(
+    val id: String,
+    val name: String,
+    val category: Type,
+    val description: String,
+    val xp: Long,
+    val goal: Long,
+    var isCompleted: Boolean,
+    val onCompletion: () -> Unit
+) {
+    enum class Type {
+        XP,
+        ACTION
+    }
+}