resolve: fix type of readFileSync (#22571)

types/resolve/index.d.ts

@@ -1,6 +1,7 @@
 // Type definitions for resolve
 // Project: https://github.com/substack/node-resolve
 // Definitions by: Mario Nebl <https://github.com/marionebl>
+//                 Klaus Meinhardt <https://github.com/ajafff>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /// <reference types="node" />
@@ -88,7 +89,7 @@ declare namespace resolve {
 
   export interface SyncOpts extends Opts {
     // how to read files synchronously (defaults to fs.readFileSync)
-    readFileSync?: (file: string) => Buffer;
+    readFileSync?: (file: string, charset: string) => string | Buffer;
     // function to synchronously test whether a file exists
     isFile?: (file: string) => boolean;
   }

types/resolve/resolve-tests.ts

@@ -73,6 +73,11 @@ function test_options_sync() {
     }
   });
   console.log(resolved);
+  resolved = resolve.sync('typescript', {
+      readFileSync(file, charset) {
+          return fs.readFileSync(file, charset);
+      }
+  });
 }
 
 function test_is_core() {