title: checkbox handling in form slice reducer
body: <p>I'm learning by building a basic form reducer setup which takes in certain user info. This is <code>formSlice.ts</code>:</p>
  <pre><code>import type { RootState } from '../store'
  
  interface FormState {
      pagesCompleted: number,
      addressStreet: string,
      addressCity: string,
  }
  
  const initialState = {
      pagesCompleted: 0,
      addressStreet: '',
      addressCity: '',
  } as FormState;
  
  export const formSlice = createSlice({
    name: 'form',
    initialState,
    reducers: {
      incrementPagesCompleted: state =&gt; {
        state.pagesCompleted += 1
      },
      setAddressStreet: (state, action: PayloadAction&lt;string&gt;) =&gt; {
        state.addressStreet = action.payload
      },
      setAddressCity: (state, action: PayloadAction&lt;string&gt;) =&gt; {
        state.addressCity = action.payload
      },
    }
  })
  
  export const { incrementPagesCompleted, setAddressStreet, setAddressCity } = formSlice.actions
  
  export default formSlice.reducer
  </code></pre>
  <p>Let's say I want to add a checkbox input for selecting hair colours (check all that apply to user, or whatever). Presumably I would need to add <code>hairColour</code> and an associated dispatch action <code>setHairColour</code> or <code>toggleHairColour</code>. How would that look in the above setup?</p>