fix(overmind): properly type operator functions

Author: christianalfoni

packages/node_modules/overmind/src/types.ts

@@ -1,5 +1,6 @@
 import { ResolveActions, ResolveState } from './internalTypes'
 import { Overmind } from './'
+import { IS_OPERATOR } from './utils'
 
 /** ===== PUBLIC API
  */
@@ -44,11 +45,14 @@ export interface IAction<ThisConfig extends IConfiguration, Value> {
 
 // We do not type operators as their low level implementation, but rather
 // how it is consumed by the developer
-export type IOperator<
+export interface IOperator<
   ThisConfig extends IConfiguration,
   Input,
   Output = Input
-> = (context: IContext<ThisConfig>, value: Input) => Output
+> {
+  (context: IContext<ThisConfig>, value: Input): Output
+  [IS_OPERATOR]: true
+}
 
 export type IDerive<
   ThisConfig extends IConfiguration,