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/answer.js

@@ -0,0 +1,9 @@
+const userName = "小周";
+let currentStage = "JavaScript 入门";
+let completedExercises = 2;
+
+completedExercises = completedExercises + 1;
+
+console.log("学习者：" + userName);
+console.log("当前阶段：" + currentStage);
+console.log("已完成练习数：" + completedExercises);