feat: add ts solution to lc problem: No.2678 (doocs#1867)

Author: thinkasany

solution/2600-2699/2678.Number of Senior Citizens/README.md

@@ -167,6 +167,12 @@ function countSeniors(details: string[]): number {
 }
 ```
 
+```ts
+function countSeniors(details: string[]): number {
+    return details.filter(v => parseInt(v.slice(11, 13)) > 60).length;
+}
+```
+
 ### **...**
 
 ```

solution/2600-2699/2678.Number of Senior Citizens/README_EN.md

@@ -157,6 +157,12 @@ function countSeniors(details: string[]): number {
 }
 ```
 
+```ts
+function countSeniors(details: string[]): number {
+    return details.filter(v => parseInt(v.slice(11, 13)) > 60).length;
+}
+```
+
 ### **...**
 
 ```