Add runtime deprecation warning for object case reducer syntax in `createSlice.extraReducers` and `createReducer`

We’d like to remove the object syntax for declaring case reducers in createReducer and createSlice in RTK 2.0. The object form was a neat trick when RTK was getting started, and admittedly played nice with action creators having an implicit toString() that returned the action type. But, the builder syntax is basically the same number of lines of code, and also provides much better type safety with TS.

In other words, this:

const todoAdded = createAction('todos/todoAdded');

createReducer(initialState, {
  [todoAdded]: (state, action) => {}
})

createSlice({
  name,
  initialState,
  reducers: {},
  extraReducers: {
    [todoAdded]: (state, action) => {}
  }
})

should be migrated to:

createReducer(initialState, builder => {
  builder.addCase(todoAdded, (state, action) => {})
})

createSlice({
  name,
  initialState,
  reducers: {},
  extraReducers: builder => {
    builder.addCase(todoAdded, (state, action) => {})
  }
})

We should set up a one-warning-per-app load deprecation notice, the way React does:

https://github.com/facebook/react/blob/d5f1b067c8bbb826b823d0354a28ba31078b70c0/packages/react/src/ReactContext.js#L44-L65

[martinerko]

hey guys, I had a quick look and I was able to implement both codemods. Added also some tests. check it out. In next PR I can add some codemod runner and README.

[phryneas]

Adding to that: the builder notation is also a lot less problematic with circular references.