Support multiple static asset dirs by allowing string arrays for `config.kit.files.assets`

Describe the problem

I'd like to get static assets from multiple locations. In this example the SvelteKit site is inside the site/ directory as part of a bigger project and I'd like to use static assets from assets/ and site/static/.

.
├── assets
│   ├── ...
└── site
    ├── src
    ├── static
    └── ...

Describe the proposed solution

config.kit.files.assets could support arrays:

// svelte.config.jg

export default {
  kit: {
    adapter: adapter(),

    files: {
      assets:[ `../assets`, `./static` ],
    },
  },
}

This currently raises

config.kit.files.assets should be a string, if specified

Alternatives considered

Create an alias of assets/ in site/static/. It works but feels hacky.

Importance

nice to have

Additional Information

No response

Start with the config.kit.files.assets validation and the static asset handling described in the issue. Check the existing configuration and asset-related tests, then verify that multiple asset directories are accepted and served without breaking the current single-directory behavior.