Use mtime as cache busting parameter

[benmccann]

Describe the problem

The config file is currently loaded with Date.now() as a cache-busting parameter. If mtime were instead used then SvelteKit could load the config with the same cache-busting parameter without us having to pass the config around which would simplify some APIs (see sveltejs/kit#5094).

Describe the proposed solution

Use mtime

Alternatives considered

1. the user imports the config
2. split the config file between svelte.config.js and vite.config.js
3. plugin intercommunication (though I don't think this would actually work)
4. await plugin creation function

Importance

would make my life easier