export the `effect` function as well from entrypoint

[jaredcwhite]

As the petite-vue distribution can't be used alongside a separate installation of @vue/reactivity, it would be helpful to gain access to the effect function via an export. Since effect is already being used internally, it doesn't increase bundle output, and effect can come in handy when using global reactive state outside of a scope.

[dschissler]

I've found that the effect function not being available makes it impossible to use Petite Vue as a driver of a generally usable custom element. It's a shame.