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/answer.js

@@ -0,0 +1,26 @@
+const student = {
+  name: "林晨",
+  stage: "JavaScript 核心",
+  sayHello() {
+    const currentName = this && this.name ? this.name : "未知调用者";
+    const currentStage = this && this.stage ? this.stage : "未知阶段";
+    console.log(`你好，我是 ${currentName}，正在学习 ${currentStage}`);
+  },
+  createArrowReporter() {
+    return () => {
+      console.log(`箭头函数继承到的 this.name：${this.name}`);
+    };
+  },
+};
+
+student.sayHello();
+
+const detachedHello = student.sayHello;
+detachedHello();
+
+const arrowReporter = student.createArrowReporter();
+arrowReporter();
+
+console.log("对象方法里的 this 通常指向调用它的对象。");
+console.log("脱离对象单独调用后，普通函数里的 this 会跟着调用方式变化。");
+console.log("箭头函数没有自己的 this，它会继承创建它时外层的 this。");