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/05-functions/answer.js

@@ -0,0 +1,21 @@
+function getAverage(score1, score2, score3) {
+  return (score1 + score2 + score3) / 3;
+}
+
+function getLevel(average) {
+  if (average >= 85) {
+    return "优秀";
+  }
+
+  if (average >= 70) {
+    return "良好";
+  }
+
+  return "继续努力";
+}
+
+const average = getAverage(88, 92, 79);
+const level = getLevel(average);
+
+console.log("平均分：", average);
+console.log("等级：", level);