…14705)

2 days ago I opened #14679 to put
to rest a gnarly DX issue with vercel.ts where it was very easy to
generate a mixed array (—> invalid config) when you had header
transforms. See that PR description for a very detailed description of
the issue.

The solution proposed in that PR was to always convert everything to the
routes format no matter what. This solves all current and future
problems related to mixed arrays, but creates a new problem because
`routes` and `rewrites`/`redirects`/`headers` have different routing
orders in different frameworks, so landing that change would change
where in the stack routes are processed.

After chatting with @mknichel, we landed on optimizing for this use
case:

```ts
export const config: VercelConfig = {
  routes: [
    routes.rewrite('/test-header', 'https://httpbin.org/headers', {
      requestHeaders: {
        authorization: `Bearer token`,
      },
    }),
    routes.redirect('/test-build', 'https://httpbin.org/headers'),
  ]
};
```

Currently this fails because the `rewrite` gets compiled to the `routes`
format since it has a header transform, but the `redirect` compiles to
the `redirects` format. So although both are placed in `routes`, it
still generates a mixed array.

This PR solves this issue. This is more reasonable than converting
everything to `routes`. It does force you to move everything into
`routes` if you have a header transform, but this is also the current
`vercel.json` behavior, so it is actually more in line with what people
expect from vercel.json.

… a conflict (#14709)

#14705 fixes this use case of
vercel.ts:

```ts
// ✅ fixed
export const config: VercelConfig = {
  routes: [
    routes.rewrite('/test-header', 'https://httpbin.org/headers', {
      requestHeaders: {
        authorization: `Bearer token`,
      },
    }),
    routes.redirect('/test-build', 'https://httpbin.org/headers'),
  ]
};
```

But the user can still have a confusing experience if they do this:

```ts
// ❌ not fixed
export const config: VercelConfig = {
  rewrites: [
    routes.rewrite('/test-env', 'https://httpbin.org/headers', {
      requestHeaders: {
        authorization: `Bearer ${deploymentEnv('API_TOKEN')}`,
      },
    }),
    routes.rewrite('/test-rewrite-regular', 'https://httpbin.org/headers')
  ],
  redirects: [
    routes.redirect('/test-build', 'https://httpbin.org/headers'),
  ]
};
```

This will fail with the error message:

```
Error: If `rewrites`, `redirects`, `headers`, `cleanUrls` or `trailingSlash` are used, then `routes` cannot be present.
```

But that's not clear because from the user's perspective they separated
their rewrites and redirects like the docs say, so where is `routes`
coming from?

We're not going to compile the `redirects` to routes
(#14679), so we need the error
message to be clearer.

This PR fails the build during config compilation if after normalization
(AKA compiling anything to `routes` that needs this transformation)
there's still a conflict. In this case, since it has the full context of
what just happened (unlike the generic vercel.json error it currently
throws) it can throw a more descriptive error message with clear
instructions for what to do.

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