feat(reroute): Allow for dynamic asynchronously rerouting

[nikosgram]

Describe the problem

The current SvelteKit Reroute functionality doesn't support dynamic rerouting for localization where settings are stored in a database. We need to implement a multi-domain, multi-prefix localization system that allows flexible domain/language combinations.

Example configuration:

• Domains: example.com, example.uk, example.gr, example.fr
• Localizations: en-gb, el-gr, en-gr, fr-fr, en-fr

Domain/prefix mappings:

el-gr:
- example.com/el-gr/
- example.gr/

en-gr:
- example.com/en-gr/
- example.gr/en/

en-gb:
- example.com/
- example.uk/

Describe the proposed solution

Modify the Reroute function to return MaybePromise<void | string> and update its implementation in server/response.js and client/client.js.

A proof-of-concept implementation: nikosgram@70d808e

I would appreciate feedback on this approach to ensure I'm not overlooking any potential issues or better alternatives.

Alternatives considered

No response

Importance

i cannot use SvelteKit without it

Additional Information

No response

[nikosgram]

An example of how this could work:

import type { Reroute } from '@sveltejs/kit';

export const reroute: Reroute = async ({ url }) => {
    if (url.pathname.startsWith('/api')) {
        return;
    }
    
    const response = await fetch(url.protocol + '//' + url.host + '/api/locales');

    const locales = await response.json(); // as Type[];

    // write re-routing logic here
};