Get more robust/user friendly with preprocessors

Is your feature request related to a problem? Please describe. Many people seem to have problems setting up preprocessors for the plugin - I guess partly because they install the extension and just expect it to work.

Describe the solution you’d like Maybe we can get more robust/user friendly by doing the following things:

• If we don’t detect a svelte.config.js, we just do a best guess by using https://github.com/kaisermann/svelte-preprocess
• If we detect that inside javascript “unexpected token”-errors occur, we enhance the error message with something like “have typed lang="typescript" in script? have you setup preprocessors correctly? see <somelink> for more info”

Describe alternatives you’ve considered Make docs clearer.

[dummdidumm]

It would mean adding it as a dependency and using it as the fallback preprocessor.

[benmccann]

I think just detecting if there are no preprocessors setup and telling them to add one with a helpful error message would be pretty helpful. It’d be a bit less magical and more explicit too, which I always think is nice