Removes the need for a dependency on svelte-preprocess in svelte-language-server, since that was all we used it for. Also tightens up the warning filter to account for situations where those could be wrong.

closes:

• Assume an empty svelte.config.js instead of requiring it? #1683 and Cannot find any of modules: sass,node-sass #1259, because the situation no longer arises (because we no longer have svelte-preprocess inside the VS Code extension)
• Remove dependency on svelte-preprocess #2391

@jasonlyu123 any situation I'm not correctly antipicating here which would make this a bad change? The only cases I see where this could have any change in behavior compared to before is if you're opening a Svelte file with lang="ts" and no config and...

• using import statements that don't adhere to verbatimModuleSyntax, but even then I think it will not make a difference because Svelte does not care which imports are left in the file.
• using decorators, because they are not transpiled and lead to a compiler error, but then the "filter out this error" logic should kick in, so it's fine