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/01-variables-and-console/starter.js

@@ -0,0 +1,9 @@
+const userName = "";
+let currentStage = "";
+let completedExercises = 0;
+
+// 任务：
+// 1. 给上面的变量赋一个合理值
+// 2. 修改一次 currentStage 或 completedExercises
+// 3. 输出 3 行学习信息
+// 例如：学习者：小周