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/06-arrays/starter.js

@@ -0,0 +1,6 @@
+const topics = ["HTML", "CSS", "JavaScript"];
+
+// 任务：
+// 1. 往 topics 里新增一个主题
+// 2. 输出 topics 的长度
+// 3. 用循环输出每一项