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/04-loops/starter.js

@@ -0,0 +1,7 @@
+// 任务：
+// 1. 用 for 输出第 1 天到第 7 天
+// 2. 第 4 天跳过，不输出
+// 3. 用 while 计算 1 到 5 的总和
+// 4. 输出 sum
+
+let sum = 0;