…uild time.

PoC works great if run from the user's svelte.config.js, but we need to extend the user config and still make it work. Currently it only works when run with the default svelte config loading.

…custom preprocessing config.

Auto-reloading doesn't work though, so that needs to be fixed, along with regex specificity.

…tions needed by sveltekit.

…internally. Also allow object configs.

Much cleaner solution, which also should be a tiny bit more performant than before.