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/11-var-and-scope/answer.js

@@ -0,0 +1,28 @@
+console.log("var 声明前：", lessonType);
+var lessonType = "JavaScript";
+
+console.log("var 声明后：", lessonType);
+// console.log("let 声明前：", chapterType);
+// let 在声明前处于暂时性死区，直接访问会报错。
+
+function compareScope() {
+  if (true) {
+    var lessonName = "变量";
+    let chapterName = "作用域";
+    const stage = "进阶";
+
+    console.log("块内：", lessonName, chapterName, stage);
+  }
+
+  console.log("块外仍能访问 var：", lessonName);
+  // console.log(chapterName);
+  // chapterName 是 let 声明的块级作用域变量，离开 if 代码块后就不能访问。
+}
+
+compareScope();
+
+const level = "核心阶段";
+console.log("const 初始值：", level);
+
+// level = "下一阶段";
+// const 不能被重新赋值，否则会报错。