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/03-operators-and-conditions/README.md

@@ -0,0 +1,28 @@
+# 练习 3：运算符和条件判断
+
+## 目标
+
+学会比较数据，并根据不同条件输出不同结果。
+
+## 你要练什么
+
+- 算术运算符
+- 比较运算符
+- 逻辑运算符
+- `if...else if...else`
+
+## 任务
+
+请完成一个“成绩评级”脚本，要求：
+
+- 根据分数计算是否及格
+- 根据分数输出等级
+- 90 分及以上为 A
+- 80 到 89 为 B
+- 60 到 79 为 C
+- 60 以下为 D
+
+## 文件
+
+- [starter.js](/Users/lijiaqing/home/wwwroot/front-end-example/03-javascript-core/03-operators-and-conditions/starter.js)
+- [answer.js](/Users/lijiaqing/home/wwwroot/front-end-example/03-javascript-core/03-operators-and-conditions/answer.js)