refactor: don't expose isPromise as an internal module (#19139)

lib/common/is-promise.ts

@@ -0,0 +1,12 @@
+export function isPromise (val: any) {
+  return (
+    val &&
+    val.then &&
+    val.then instanceof Function &&
+    val.constructor &&
+    val.constructor.reject &&
+    val.constructor.reject instanceof Function &&
+    val.constructor.resolve &&
+    val.constructor.resolve instanceof Function
+  )
+}