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/03-operators-and-conditions/starter.js

@@ -0,0 +1,9 @@
+const score = 86;
+
+// 任务：
+// 1. 用布尔值保存是否及格
+// 2. 用 if...else if...else 判断等级
+// 3. 输出分数、是否及格、等级
+
+let passed = false;
+let grade = "";