Svelte 5: client-side render function

[Rich-Harris]

Describe the problem

Per #12512 (comment): it's common in a programmatic snippet to do render(SomeComponent, props).body, but this doesn't work in the browser because SomeComponent is compiled in a way that doesn't work with render.

Describe the proposed solution

Either or both of:

• a browser version of render that behaves differently in some way (at minimum a more helpful error, but maybe it could return a body string that tells the component that subsequently mounts how to do so)
• a svelte/environment module akin to $app/environment, providing access to browser and dev

Of course, in an ideal world it would Just Work, but that would require shipping the server-side templating logic to the client, unless there's some clever tree-shaking trickery we can employ.

Importance

nice to have