Remove support of svelte preprocessor settings

[samuelgozi]

🙋 Feature Request

Detailed description

I don't think there is any need to pass options to or run the Svelte preprocessor.
As stated in the official docs for Svelte, it is mainly intended for transpilation, for example of less to css.
But since parcel itself can take care of this kind of stuff, I see no reason for letting svelte do it instead, In addition, it might be counterproductive performance-wise to let it do it.

We should just pass the assets generated from the compilation to parcel.
I can do the refactor if it is accepted.

[samuelgozi]

After doing some research it seems like its not a good idea at this time.
Mainly because if someone wants to write any other "flavor" of js, like for example typescript, the preprocessor would convert it to JS, and then return it to the svelte compiler since it needs the file for compilation.

I'm not very familiar with Parcel's internals, but I don't know a way or letting parcel handle some file(for example typescript), and then return it to the svelte compiler to finish the compilation.

So this means that for some cases, the preprocessor will still be relevant, or even the only way of making it work properly.