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/06-arrays/answer.js

@@ -0,0 +1,9 @@
+const topics = ["HTML", "CSS", "JavaScript"];
+
+topics.push("DOM");
+
+console.log("学习主题数量：", topics.length);
+
+for (let index = 0; index < topics.length; index += 1) {
+  console.log(`第 ${index + 1} 项：${topics[index]}`);
+}