Add context typings

[jpavon]

Related issue #5

[tsiq-swyx]

this is great! thank you! I might have to reorganize a bit, i'll do this after merge

[slorber]

note this implementation is not recommended for perf reasons, as creating a new store object on every render means that re-rendering the provider will make all the consumers re-render.

public render() {
    const store: ProviderStore = {
      state: this.state,
      update: this.update
    }

    return (
      <Context.Provider value={store}>
        {this.props.children}
      </Context.Provider>
    )
  }

This is one reason I built https://github.com/slorber/react-updatable-context, which is somehow an inferior version of Unstated