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/12-this-keyword/starter.js

@@ -0,0 +1,18 @@
+const student = {
+  name: "林晨",
+  stage: "JavaScript 核心",
+  sayHello() {
+    // 任务：
+    // 1. 用 this.name 输出问候语
+  },
+  createArrowReporter() {
+    // 任务：
+    // 2. 返回一个箭头函数
+    // 3. 在箭头函数里输出 this.name
+  },
+};
+
+// 任务：
+// 4. 调用 student.sayHello()
+// 5. 把 student.sayHello 赋值给 detachedHello 再调用
+// 6. 调用 createArrowReporter 返回的新函数