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/08-built-in-methods/starter.js

@@ -0,0 +1,8 @@
+const rawTitle = "  js 核心练习  ";
+const tags = ["变量", "条件", "函数"];
+
+// 任务：
+// 1. 去掉 rawTitle 两端空格
+// 2. 转成大写
+// 3. 判断是否包含 JS
+// 4. 把 tags 用顿号连接成一句话