Add overload that returns promise (#38864)

* Add overload that returns promise

which@2.0.1 supports `Promise`

* Add test for the promise variant

* Add promise overloads with options

* Update which-tests.ts

* Add missing ';'

types/which/index.d.ts

@@ -13,6 +13,10 @@ declare function which(cmd: string, options: which.AsyncOptions & which.OptionsF
 declare function which(cmd: string, options: which.AsyncOptions, cb: (err: Error | null, path: string | Array<string> | undefined) => void): void;
 /** Finds the first instance of a specified executable in the PATH environment variable */
 declare function which(cmd: string, cb: (err: Error | null, path: string | undefined) => void): void;
+/** Finds the first instance of a specified executable in the PATH environment variable */
+declare function which(cmd: string, options: which.AsyncOptions & which.OptionsAll): Promise<string[]>;
+/** Finds the first instance of a specified executable in the PATH environment variable */
+declare function which(cmd: string, options?: which.AsyncOptions & which.OptionsFirst): Promise<string>;
 declare namespace which {
     /** Finds all instances of a specified executable in the PATH environment variable */
     function sync(cmd: string, options: which.Options & which.OptionsAll & which.OptionsNoThrow): Array<string> | null;

types/which/which-tests.ts

@@ -17,6 +17,10 @@ which("cat", {all: true}, (err, paths) => {
   }
 });
 
+var promise: Promise<string> = which("cat");
+var promise1: Promise<string> = which("cat", { all: false });
+var promise2: Promise<string[]> = which("cat", { all: true });
+
 var paths = which.sync("cat", {all: true});
 for(let path of paths) {
   console.log(path);