Add ASCII output support

I've been working on a product using libsass/sassc and have migrated it to Dart Sass. Presently, this product doesn't support UTF-8 characters in stylesheets.

It looks like Dart Sass only supports outputting as UTF-8, with dart-lang/sdk#11744 being the blocker given in the README for why there's no support for more encodings (UTF-16, etc). Dart does however appear to have an AsciiEncoder.

For the time being, we've added an extra step to CSS escape non-ASCII characters in generated stylesheets. (On a related note, we also remove the @charset 'UTF-8'; atrule, both because it would be technically incorrect for an ASCII-encoded stylesheet, and because of #567.)

This isn't trivial because of sourcemaps, so we're doing this step with a PostCSS plugin.

Would adding ASCII-encoded output support be in scope of Dart Sass? I'd imagine when Dart adds other encoders, having this ready would let other encodings be sibling output options alongside UTF-8 and ASCII.