New "strict order" option for preprocessor

[matyunya]

Original discussion: sveltejs/rollup-plugin-svelte#86 (comment).

Issue: #4141

New option allows users to specify strict order in which preprocessors must be applied, specifically for the case when Sass preprocessor must run before Svelte image to work properly.

[Conduitry]

What happens when you enable strict ordering and some of the objects in the array of preprocessors contain multiple types of preprocessors in the same object? I'm not positive what I would want to happen. Run markup and then the other two? Throw an exception?

[matyunya]

Since we're allowing users to specify only the order of preprocessor groups inside the group the behaviour remains the same, seems the best way to me:

if (p.markup) {
			processed = await process_markup(source, p.markup, filename);
			source = processed.source;
if (p.script) {
			processed = await process_script(source, p.script, filename);
			source = processed.source;
if (p.style) {
			processed = await process_style(source, p.style, filename);
			source = processed.source;