closes sveltejs/language-tools#2321

This is a breaking change as it adds a peer dependency on vite-plugin-svelte for resolving the Svelte config file

This changes the implementation from a preprocessor to a plugin with enforce: 'pre' to help vite-plugin-svelte kill off it's preprocessing API. The code change looks pretty big as a result of the file rename, but it's not actually a huge change

I'm removing the warning from the docs about the plugin being unstable as I don't anticipate major API changes now that we've decided against renaming this to something like <img data-enhanced />, etc.