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/answer.js

@@ -0,0 +1,17 @@
+for (let day = 1; day <= 7; day += 1) {
+  if (day === 4) {
+    continue;
+  }
+
+  console.log(`第 ${day} 天：完成学习打卡`);
+}
+
+let current = 1;
+let sum = 0;
+
+while (current <= 5) {
+  sum += current;
+  current += 1;
+}
+
+console.log("1 到 5 的总和：", sum);