Add ASCII output support

[rzhw]

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.

[nex3]

This is something I could see adding as a command-line flag (--ascii-only or something like that) to serialize Unicode characters as ASCII escapes.

[bit-wise]

@nex3 will this command-line flag be ready when Ruby sass is deprecated?

[nex3]

Ruby Sass has been deprecated for almost a year now. And no, there's no plan to tie this issue to its release cycle. It's marked as "help wanted", which means it's not a priority for the Sass team, but if an external user wanted to contribute a fix we'd help them land it.

[jpuncle]

@Awjin When to solve this problem?

[BuptStEve]

// @source - [@Stephn-R](https://github.com/sass/sass/issues/1395#issuecomment-57483844)
// @description converts 1 or more characters into a unicode
// @markup {scss}
// unicode("e655"); // "\e655"
@function unicode($str){
    @return unquote("\"")+unquote(str-insert($str, "\\", 1))+unquote("\"")
}