Use string literal type for script kind names

src/server/protocol.d.ts

@@ -520,9 +520,9 @@ declare namespace ts.server.protocol {
         fileContent?: string;
         /**
          * Used to specify the script kind of the file explicitly. It could be one of the following:
-         *      ".ts", ".js", ".tsx", ".jsx"
+         *      "TS", "JS", "TSX", "JSX"
          */
-        scriptKindName?: string;
+        scriptKindName?: "TS" | "JS" | "TSX" | "JSX";
     }
 
     /**