Only apply svelte-123xyz attributes where necessary

[Rich-Harris]

Making a start on #679 (and laying the groundwork for #678). Have hit a bit of a snag — the work needs to happen in preprocess, because of descendant selectors:

<div>
  <p>both this element and its parent need the scoping attribute</p>
</div>

<div>
  <strong>neither of these two need the attribute</strong>
</div>

That's going to cause some nasty merge conflicts unless I merge #673 and #676, so I'll do that first.

[Rich-Harris]

This is almost done — think I just need to implement selectorAppliesTo for universal TypeSelector (*), ID selectors, attribute selectors, and possibly ~ and + combinators (though might just bail for now in those cases)

[Rich-Harris]

Taking the WIP off the title, in case anyone fancies taking a look!

[Conduitry]

selectors: ((nodes: Node[]) => boolean)[]; I believe

edit: Although this type doesn't seem to match how it's used later. extractSelectors returns simply Node[].

[Rich-Harris]

Good catch. Have refactored it a bit so that it's a bit more TS-friendly (and possibly a bit clearer anyway) — a Selector class with an apply method

[Rich-Harris]

Did a bit more futzing around after merging #689 and concluded that we could use a more comprehensive refactoring of the CSS code, which is currently a bit scattered and woolly. In particular, it would be nice to support CSS sourcemaps (and maybe in future, CSS from multiple sources i.e. @import and <link>), which would probably be a bit of a pain to implement at the moment. Will return to that after fixing the bugs that have been raised recently.