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

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+# 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)

03-javascript-core/03-operators-and-conditions/answer.js

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+const score = 86;
+
+let passed = score >= 60;
+let grade = "";
+
+if (score >= 90) {
+  grade = "A";
+} else if (score >= 80) {
+  grade = "B";
+} else if (score >= 60) {
+  grade = "C";
+} else {
+  grade = "D";
+}
+
+console.log("分数：", score);
+console.log("是否及格：", passed);
+console.log("等级：", grade);

03-javascript-core/03-operators-and-conditions/starter.js

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+const score = 86;
+
+// 任务：
+// 1. 用布尔值保存是否及格
+// 2. 用 if...else if...else 判断等级
+// 3. 输出分数、是否及格、等级
+
+let passed = false;
+let grade = "";