support images and fonts url() in css

[Fil]

closes #786

here's an example application (which I wouldn't necessarily recommend!) with a background image from docs/assets/ and fonts saved in docs/fonts/:

:root {
  --serif: Cardo, serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
  background-image: url("assets/paper-texture.jpeg");
}

@font-face {
  font-family: "Cardo";
  src: url("fonts/cardo.woff") format("woff");
  font-display: swap;
}

You can also use static assets to modify the toggle or caret with the docs/toggle.svg file :

:root {
  --theme-toggle: url("toggle.svg");
}

The resources are inlined in text (for svg) base64 (for binary images and fonts), so use with some caution. Note that inlining fonts avoids FOUC, and the style.css is cached, so it's not necessarily bad. Just put these definitions at the bottom of stylesheet? The alternative is to use absolute URLs to an assets server. I'm working on saving these as assets.

[mbostock]

Hmm, I do think we’ll want to figure out how to treat these as assets that get included in the build (and added to _file or _import and content-hashed so it can be immutably cached). We should be able to mark these as external and then add them to the build, rewrite the relative paths, somehow?

[Fil]

related #423

[mbostock]

We should change this so that importing in CSS produces the same result as linking to a static asset in HTML (i.e., it goes in _file).

[Fil]

superseded by #1372 😓