[promise.prototype.finally] add typings for the non-shim variant (#18114)

* [promise.prototype.finally] add typings for the non-shim variant of promise.prototype.finally

* [promise.prototype.finally] change indentation to 4 spaces

* [promise.prototype.finally] fix lint error

types/promise.prototype.finally/index.d.ts

@@ -1,6 +1,7 @@
 // Type definitions for promise.prototype.finally 2.0
 // Project: https://github.com/matthew-andrews/Promise.prototype.finally
 // Definitions by: Slava Shpitalny <https://github.com/slavik57>
+//                 BendingBender <https://github.com/BendingBender>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 declare global {
@@ -9,4 +10,9 @@ declare global {
     }
 }
 
-export function shim(): void;
+export = promiseFinally;
+
+declare function promiseFinally<T, U>(promise: Promise<T>, onFinally?: () => U | PromiseLike<U>): Promise<T>;
+declare namespace promiseFinally {
+    function shim(): void;
+}

types/promise.prototype.finally/promise.prototype.finally-tests.ts

@@ -3,7 +3,7 @@ import promiseFinally = require('promise.prototype.finally');
 promiseFinally.shim();
 
 let promise = new Promise<boolean>((resolve, reject) => {
-	resolve(true);
+    resolve(true);
 });
 
 promise.finally(() => {});
@@ -13,3 +13,20 @@ promise.catch(() => {}).finally(() => {});
 
 let allPromise = Promise.all([promise]);
 allPromise.finally(() => {});
+
+const resolved = Promise.resolve(42);
+const rejected = Promise.reject(-1);
+
+let num: number;
+promiseFinally(resolved, () => Promise.resolve(true))
+  .then(x => num = x);
+
+promiseFinally(resolved, () => <PromiseLike<boolean>> Promise.resolve(true))
+  .then(x => num = x);
+
+promiseFinally(rejected, () => false)
+  .catch(e => num = e);
+
+let bool: boolean;
+promiseFinally(rejected, () => {throw false; })
+  .catch(e => bool = e);