feat(Kotlin): Replace value with _val (#1254)

* ci(kotlin): Add workflow file.

* Update kotlin.yml

* style(kotlin): value -> _val

---------

Co-authored-by: Yudong Jin <krahets@163.com>

codes/kotlin/chapter_tree/binary_tree_dfs.kt

@@ -16,7 +16,7 @@ var list = mutableListOf<Int>()
 fun preOrder(root: TreeNode?) {
     if (root == null) return
     // 访问优先级：根节点 -> 左子树 -> 右子树
-    list.add(root.value)
+    list.add(root._val)
     preOrder(root.left)
     preOrder(root.right)
 }
@@ -26,7 +26,7 @@ fun inOrder(root: TreeNode?) {
     if (root == null) return
     // 访问优先级：左子树 -> 根节点 -> 右子树
     inOrder(root.left)
-    list.add(root.value)
+    list.add(root._val)
     inOrder(root.right)
 }
 
@@ -36,7 +36,7 @@ fun postOrder(root: TreeNode?) {
     // 访问优先级：左子树 -> 右子树 -> 根节点
     postOrder(root.left)
     postOrder(root.right)
-    list.add(root.value)
+    list.add(root._val)
 }
 
 /* Driver Code */