feat(signal-slice): add signalSlice

[joshuamorony]

This was created in collaboration with Chau, it adds signalSlice which is loosely inspired by the createSlice API from Redux Toolkit. The general idea is that it allows you to declaratively create a "slice" of state. This state will be available as a readonly signal.

The key motivation, and what makes this declarative, is that all the ways for updating this signal are declared upfront with sources and reducers. It is not possible to imperatively update the state.

This PR also makes minor changes to connect in order to expose types that are shared by this utility.

Example using all API features:

  state = signalSlice({
    initialState: this.initialState,
    sources: [this.sources$],
    reducers: {
      add: (state, checklist: AddChecklist) => ({
        checklists: [...state.checklists, this.addIdToChecklist(checklist)],
      }),
      edit: (state, update: EditChecklist) => ({
        checklists: state.checklists.map((checklist) =>
          checklist.id === update.id
            ? { ...checklist, title: update.data.title }
            : checklist
        ),
      }),
      remove: (state, id: RemoveChecklist) => ({
        checklists: state.checklists.filter((checklist) => checklist.id !== id),
      }),
    },
    selectors: (state) => ({
      loadedAndError: () => state().loaded && state().error,
      whatever: () => 'hi',
    }),
  });

Key features:

• Sources will automatically update signal when they emit
• Reducers will automatically create actions on the state object that can be called (e.g. this.state.remove(id))
• Top-level properties of the initial state are automatically exposed as selectors (computeds) on the state object (e.g. this.state.loaded()`)
• Additional selectors can be supplied which will also be added to the state object

For my own purposes this API is "feature complete" and does everything I need but I'm very open to comments/revisions/improvements.

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[nartc]

Some nits but approved

[nartc]

@all-contributors please add @joshuamorony for code

[allcontributors]

@nartc

I've put up a pull request to add @joshuamorony! 🎉