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/answer.js

@@ -0,0 +1,18 @@
+const score = 86;
+
+let passed = score >= 60;
+let grade = "";
+
+if (score >= 90) {
+  grade = "A";
+} else if (score >= 80) {
+  grade = "B";
+} else if (score >= 60) {
+  grade = "C";
+} else {
+  grade = "D";
+}
+
+console.log("分数：", score);
+console.log("是否及格：", passed);
+console.log("等级：", grade);