add orderPreprocessors option

[matyunya]

This addresses the specific case when Sass preprocessor needs to be applied before Svelte Image (style before markup), because Svelte parser throws on encountering sass syntax in the style section.

[atwoodhouse]

The option orderPreprocessors should probably be renamed since it's a bit counter-intuitive. What it does is that it tells rollup-plugin-svelte NOT to reorder the preprocessors but apply them in exactly the order we passed.

Maybe strictOrder: true or reorder: false (with true being the default) would be clearer.

Apart from the renaming I really vote for this being merged into rollup-plugin-svelte since it's critical to be able to apply style preprocessors before markup preprocessors in some cases such as this: https://github.com/atwoodhouse/example-svelte-multiple-preprocessors/
For reference, the discussion leading up to this PR: sveltejs/svelte#4141

[matyunya]

I think strictOrder sounds better, renamed the option.

[Conduitry]

I think I would prefer to have this option added to the underlying svelte.preprocess() function, so that it could be used in any build system. We'd still need to update this plugin to pass along the value of the option, but none of the rest of the work here seems to be specific to Rollup.

It also doesn't look like this code would populate the dependencies array when strictOrder is enabled.

[matyunya]

Here's the PR: sveltejs/svelte#4222.