fix: add jsdoc in tree.js, avoiding codesmells (#3476)

Author: Uzlopak

lib/core/tree.js

@@ -47,6 +47,9 @@ class TstNode {
       throw new TypeError('Unreachable')
     }
     let index = 0
+    /**
+     * @type {TstNode}
+     */
     let node = this
     while (true) {
       const code = key.charCodeAt(index)
@@ -87,6 +90,9 @@ class TstNode {
   search (key) {
     const keylength = key.length
     let index = 0
+    /**
+     * @type {TstNode}
+     */
     let node = this
     while (node !== null && index < keylength) {
       let code = key[index]