React Renderer shouldn't require a separate components file

[mikehearn]

Feature description

Currently the React Renderer expects you to compile one or more bundles of components, and also lets you optionally supply a custom render script. The idea is that render scripts are generic, whereas the bundles are app specific.

It turns out that some users find it easier to just create a render script that does everything rather than use the split mechanism. In that case you have to create a fake bundle, which is a bit silly. It'd be better if we made the ssr-components.mjs file optional.