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/09-scope-and-closure/answer.js

@@ -0,0 +1,16 @@
+function createCounter() {
+  let count = 0;
+
+  return function () {
+    count += 1;
+    return count;
+  };
+}
+
+const counterA = createCounter();
+const counterB = createCounter();
+
+console.log("counterA 第一次：", counterA());
+console.log("counterA 第二次：", counterA());
+console.log("counterB 第一次：", counterB());
+console.log("每次调用 createCounter 都会创建一个新的闭包环境。");