Add a promises package with a MakeHandledPromise helper.

This helper function makes it convenient to create a promise in the
context of  k6 module/extension Go code.

js/promises/promises.go

@@ -0,0 +1,46 @@
+// Package promises provides helpers for working with promises in k6.
+package promises
+
+import (
+	"github.com/dop251/goja"
+	"go.k6.io/k6/js/modules"
+)
+
+// MakeHandledPromise can be used to create promises that will be dispatched to k6's event loop.
+//
+// Calling the function will create a goja promise and return its `resolve` and `reject` callbacks, wrapped
+// in such a way that it will block the k6 JS runtime's event loop from exiting before they are
+// called, even if the promise isn't revoled by the time the current script ends executing.
+//
+// A typical usage would be:
+//
+//	   func myAsynchronousFunc(vu modules.VU) *(goja.Promise) {
+//		    promise, resolve, reject := promises.MakeHandledPromise(vu)
+//		    go func() {
+//		        v, err := someAsyncFunc()
+//				   if err != nil {
+//		            reject(err)
+//		            return
+//		        }
+//
+//		        resolve(v)
+//		    }()
+//		    return promise
+//		  }
+func MakeHandledPromise(vu modules.VU) (*goja.Promise, func(interface{}), func(interface{})) {
+	runtime := vu.Runtime()
+	callback := vu.RegisterCallback()
+	promise, resolve, reject := runtime.NewPromise()
+
+	return promise, func(i interface{}) {
+			callback(func() error {
+				resolve(i)
+				return nil
+			})
+		}, func(i interface{}) {
+			callback(func() error {
+				reject(i)
+				return nil
+			})
+		}
+}