Add type annotation into services so _services_ API baseline is stable post-LKG (microsoft#42028)

Author: weswigham

src/server/scriptInfo.ts

@@ -559,7 +559,7 @@ namespace ts.server {
             }
         }
 
-        getLatestVersion() {
+        getLatestVersion(): string {
             // Ensure we have updated snapshot to give back latest version
             this.textStorage.getSnapshot();
             return this.textStorage.getVersion();