allow `config.kit.files.routes` to be an array of directories

Describe the problem

We have a multi-tenant SvelteKit project, with a directory structure like following:

src/routes/
 - index.svelte
 - about.svelte
 - terms.svelte
 - [...project1]/
    - index.svelte
 - [...project2]/
    - about.svelte

Here, we've many common routes (index, about, terms). Each project can override the common routes with a few of their custom routes (like project1 has its own index.svelte but uses the common about.svelte; and project2 has a different about.svelte but uses the common index and terms page.

Before we had the new routing system (https://github.com/sveltejs/kit/discussions/5774), we could filter out specific files from routes directory to be considered for routing using the config.kit.routes function. This was done based on build-time env variables like following:

   routes(filepath) {
      const currentProject = process.env.PROJECT_ID;
      // Exclude special folder that doesn't match current project.
      const re = /^\/\[\.\.\.(project1|project2)\]/;
      if (re.test(filepath) && filepath.match(re)[1] !== currentProject) {
        return false;
      }

      // If we've a matching route in currentProject-specific folder, exclude the common
      // filepath route. We'll end up using currentProject-specific one as `params[currentProject]` can
      // be an empty string, which SvelteKit router matches.
      // This does mean `/anything/foo` as well as `/foo resolve to same page, 
      // but we can exclude `/{anything}/foo` using route params. Hacky, but works.
      const specialRoute = path.join(`src/routes/[...${currentProject}]`, filepath);
      return !existsAsFile(specialRoute);
   },

Now, the config.kit.routes option has been removed and we can't upgrade to newer versions without major changes to our approach.

Describe the proposed solution

• Bring back config.kit.routes option.
• Support multiple directory in config.kit.files.routes, which can support overrides like above. This is arguably a niche use-case, but would love if this can be supported.

Alternatives considered

We can create multiple top-level routes directories and symlink/hardlink the common routes to project-specific directory, while avoiding the existing non-symlink project-specific files, to have a directory structure like:

src/
  - routes.common/
    - index.svelte
    - about.svelte
    - terms.svelte
  - routes.project1/
    - index.svelte
    - about.svelte <symlink>
    - terms.svelte <symlink>
  - routes.project2/
    - index.svelte <symlink>
    - about.svelte
    - terms.svelte <symlink>

But this might cause confusion, where we update one file and it unexpectedly ends up updating other file too. Also, it will require us to delete the symlink and create a regular file when we want to override a route.

Importance

would make my life easier

Additional Information

No response

Start by reviewing the config.kit.files.routes option and the removed config.kit.routes behavior, then read routing-system discussion #5774 for context. Define how multiple route directories and project-specific overrides should behave, and verify that common routes can be selected alongside matching project routes without the old filtering option.