json-file-plus: fix types (DefinitelyTyped#41988)

Some of these types were wrong, and some were missing.

package.json

@@ -31,5 +31,8 @@
             "_comment": "This will remove husky from when we started migrating to use prettier",
             "pre-commit": "npm uninstall husky"
         }
+    },
+    "dependencies": {
+        "@types/node": "^13.7.0"
     }
 }

types/json-file-plus/index.d.ts

@@ -3,23 +3,26 @@
 // Definitions by: Daniel Byrne <https://github.com/danwbyrne>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
+/// <reference types="node" />
+
 declare function readJSON(filename: string): Promise<readJSON.JSONFile>;
 
 declare namespace readJSON {
   function sync(filename: string): JSONFile;
 
   interface Format {
-    readonly indent: number;
+    readonly indent: number | '\t';
     readonly trailing: boolean;
   }
 
   class JSONData {
     format: Format;
     data: any;
     constructor(raw: string);
-    get: (key: string) => Promise<any>;
+    get: (key: PropertyKey) => Promise<any>;
     set: (data: any) => void;
-    remove: (key: string) => Promise<void>;
+    remove: (key: PropertyKey) => Promise<void>;
+    stringify: () => Buffer;
   }
 
   class JSONFile extends JSONData {