Overloading of mutation methods is not correctly typed in Actions

[vdkrasny]

First of all, thank you, @ktsn, for a great library! I've been using it for over half a year and it's extremely useful 👍

I've prepared a small example to demonstrate the issue:

import { Getters, Mutations, Actions } from 'vuex-smart-module';

export class TestState {}

export class TestGetters extends Getters<TestState> {}

export class TestMutations extends Mutations<TestState> {
  methodWithOverload(): void;
  methodWithOverload(payload: number): void;
  methodWithOverload(payload: string): void;
  methodWithOverload(payload?: number | string): void {
    // ...
  }

  testMutation() {
    // (method) TestMutations.methodWithOverload(): void (+2 overloads)
    this.methodWithOverload();
    this.methodWithOverload(1);
    this.methodWithOverload('test');
  }
}

export class TestActions extends Actions<TestState, TestGetters, TestMutations, TestActions> {
  testAction() {
    // (method) methodWithOverload(payload?: string | undefined): void
    this.mutations.methodWithOverload();
    this.mutations.methodWithOverload(1); // <-- Argument of type '1' is not assignable to parameter of type 'string | undefined'.
    this.mutations.methodWithOverload('test');
    
    
    this.commit('methodWithOverload');
    this.commit('methodWithOverload', 1); // <-- Argument of type 'number' is not assignable to parameter of type 'string'.
    this.commit('methodWithOverload', 'test');
  }
}

As you can see, when we call methodWithOverload from Actions, its type is not the same as the original.

• vue: 2.6.11
• vuex: 3.4.0
• vuex-smart-module: 0.4.5
• typescript: 3.9.3