feat(effect): add effect to public api

[RobbinBaauw]

For writing custom renderers it would be nice to be able to follow a similar structure as found in the base renderer:
https://github.com/vuejs/vue-next/blob/5c33776e67a5136aca5100f8ad1b0c968527f8de/packages/runtime-core/src/renderer.ts#L1095-L1097

This would require having the effect function available to use. We can't import it from the reactivity package because of the following snippet from the readme:

This package is inlined into Global & Browser ESM builds of user-facing renderers (e.g. @vue/runtime-dom), but also published as a package that can be used standalone. The standalone build should not be used alongside a pre-bundled build of a user-facing renderer, as they will have different internal storage for reactivity connections. A user-facing renderer should re-export all APIs from this package.

This PR adds this function to the public API.

[yyx990803]

This is reverted because I misread my own explanation. If you are building a user-facing renderer, you should have both @vue/reactivity and @vue/runtime-core in your dependencies, and be able to import effect from @vue/reactivity.

The sentence

The standalone build should not be used alongside a pre-bundled build of a user-facing renderer, as they will have different internal storage for reactivity connections.

only applies to the situation where:

1. You are shipping your custom renderer with @vue/reactivity inlined;
2. A consumer of your custom renderer decides to install @vue/reactivity as a standalone dependency and import effect from it.

Which should be really rare, since end users of custom renderers shouldn't need effect. Or, if you think your users would need it, you can export effect from your custom renderer.