Add support for <link> elements in svelte components

[tivac]

Instead of only supporting single file component <style>, this adds support for <link> tags that reference an external stylesheet. Part of the impetus behind this is making transitioning from the old-style import css from "./styles.css"; flow easier to transition away from. Before you'd need to copy the contents of the CSS file into the svelte component. Now the process is deleting some code from inside the <script> block and turning the import into a <link>.

It also makes referencing classes/values from component-level CSS possible, because modular-css already knows how to process a CSS file. It doesn't know anything about a svelte HTML file though & it'd be a ton of work to add support for that.

So now there's an easy transitional path from the old-style css-as-data model to the preprocessor world where most classes can be completely static!