Export types from /server entrypoint

[rdsfj]

Hello 🙋‍♀️

I'm using single-spa-layout and it works great so far!
One thing I'm missing though is that the entrypoint single-spa-layout/server doesn't export any types, but just its JS API.
I've been using a patch and patch-package for some time now, so I thought I could contribute the changes.

I'd follow the pattern that's already there for the main entrypoint:

• Create single-spa-layout-server-interface.ts with additional type exports
• Export types used in the public API: HTMLTemplateOptions, ServerLayout, AppToRender, ApplicationRenderResult, RenderResult.
• Point the types field in package.json to the new file.

Would you accept a PR for these changes?

Best regards
Simon