Fallback for SPA should support files other than index.html

Description

Currently, there is the following option (mentioned in #664):

-s, --single                        Rewrite all not-found requests to `index.html`

However, this option is limited and does not support loading custom HTML as mentioned by @woss in:

• https://github.com/vercel/serve/issues/695#issuecomment-2047172879

Why

Frameworks like Tanstack Start generates _shell.html by default for SPA, rather than index.html:

• https://github.com/TanStack/router/discussions/3394

• https://tanstack.com/start/v0/docs/framework/react/guide/spa-mode#how-does-it-work

  

Alternatives

A workaround can be seen here:

• https://github.com/fuma-nama/fumadocs/pull/3378

Where serve.json is created with a rewrite:

{
  "rewrites": [
    {
      "source": "!/api/**",
      "destination": "/_shell.html"
    }
  ]
}

This isn't ideal, however, since the rewrite works on all paths rather than serving as a fallback, which masks the API route - thus, it needs to be excluded in the source glob. There may also be other exclusions needed apart from /api.