Generate per-route redirects in Vercel build output instead of per-page

Describe the problem

In order to get the correct trailing slash redirect behaviour for prerendered pages on Vercel, it's necessary to add an override plus two routes for each page:

{
  "version": 3,
  "routes": [
    // ...
    {
      "src": "/docs/introduction",
      "dest": "/docs/introduction/"
    },
    {
      "src": "/docs/introduction/",
      "status": 308,
      "headers": {
        "Location": "/docs/introduction"
      }
    },
    {
      "src": "/docs/configuration",
      "dest": "/docs/configuration/"
    },
    {
      "src": "/docs/configuration/",
      "status": 308,
      "headers": {
        "Location": "/docs/configuration"
      }
    },
    // ...
  ],
  "overrides": {
    // ...
    "docs/introduction.html": {
      "path": "docs/introduction"
    },
    "docs/configuration.html": {
      "path": "docs/configuration"
    },
    // ...
  }
}

Since there's a limit to how many routes you can have in a project, this means that there's also a (quite low) limit on the number of prerendered pages.

Describe the proposed solution

We should be able to create a pair of Vercel redirect routes for each SvelteKit route, instead of for each prerendered page:

{
  "version": 3,
  "routes": [
    // ...
    {
      "src": "/docs/([^/]+)",
      "dest": "/docs/$1/"
    },
    {
      "src": "/docs/([^/]+)/",
      "status": 308,
      "headers": {
        "Location": "/docs/$1"
      }
    },
    // ...
  ],
  "overrides": {
    // ...
    "docs/introduction.html": {
      "path": "docs/introduction"
    },
    "docs/configuration.html": {
      "path": "docs/configuration"
    },
    // ...
  }
}

(I actually can't recall why the first of those is necessary in the trailingSlash: 'never' case, need to investigate.)

The challenge will be to do this while respecting edge cases like this...

src/routes/[a=b]/+page.svelte
src/routes/[c]/+page.svelte

...where the two routes have different trailingSlash config and /[a=b] is a prerendered route that shouldn't prevent /[c] from matching.

Alternatives considered

No response

Importance

would make my life easier

Additional Information

No response

Start by tracing how Vercel build output generates per-prerendered-page redirects and overrides, using the route examples in src/routes/[a=b]/+page.svelte and src/routes/[c]/+page.svelte as edge cases. Investigate why the first redirect is needed for trailingSlash: 'never'. Done means generating per-SvelteKit-route redirect pairs while preserving distinct trailingSlash settings and correct matching for prerendered routes.