feat: Add JavaScript core exercises and solutions

- Implemented exercises for array high-order methods, memory and execution, switch statements, and final review.
- Added starter and answer files for each exercise to facilitate learning.
- Created a runner HTML file to execute JavaScript code and display console outputs.
- Updated README files to include exercise objectives, tasks, and usage instructions.

03-javascript-core/09-scope-and-closure/starter.js

@@ -0,0 +1,11 @@
+function createCounter() {
+  let count = 0;
+
+  // 返回一个函数
+}
+
+// 任务：
+// 1. 创建 counterA 和 counterB
+// 2. 连续调用 counterA 两次
+// 3. 再调用 counterB 一次
+// 4. 观察为什么两个计数器互不影响