Add typings

[jonaskello]

These typings build on the typings from the PR in the raj package and provide some context for further discussion.

[jonaskello]

Specifically this part which closes the generic V in the Program type to provide a ReactProgram type that has a View that return ReactNode. Not sure if it is needed or makes sense, but it is the way I did it when I tried out raj.

export type ReactProgram<S, M> = Program<S, M, React.ReactNode>;