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/README.md

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+# 练习 8：常见内置方法
+
+## 目标
+
+学会使用几个常见的字符串和数组方法，减少重复代码。
+
+## 你要练什么
+
+- `trim()`
+- `toUpperCase()`
+- `includes()`
+- `join()`
+
+## 任务
+
+请完成一个“标签清洗器”脚本，要求：
+
+- 把一段带空格的文本去掉首尾空格
+- 把结果转成大写
+- 判断里面是否包含 `JS`
+- 把一个标签数组拼成一个字符串输出
+
+## 文件
+
+- [starter.js](/Users/lijiaqing/home/wwwroot/front-end-example/03-javascript-core/08-built-in-methods/starter.js)
+- [answer.js](/Users/lijiaqing/home/wwwroot/front-end-example/03-javascript-core/08-built-in-methods/answer.js)