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/starter.js

@@ -0,0 +1,11 @@
+function getAverage(score1, score2, score3) {
+  // 返回平均分
+}
+
+function getLevel(average) {
+  // 根据平均分返回等级描述
+}
+
+// 任务：
+// 1. 调用上面两个函数
+// 2. 输出平均分和等级