Migrate to latest Sass syntax

[RobinvanderVliet]

I ran the following command in the folder src: sass-migrator module flagpack.scss

Please check whether this fully solves the issue.

Fixes #14

[timobrembeck]

First of all, sorry for the long silence — this PR deserved a response much sooner. Thanks a lot for the migration to the Sass module system! The change compiles cleanly and the generated CSS is byte-identical to the current dist/flagpack.css. 🎉

One side effect of @use: the customization pattern documented in the README (defining variables like $fp-prefix before importing) no longer has any effect, because module variables can only be configured via @use ... with (...). I've pushed a small commit to your branch that updates the README's SASS section with the new pattern:

@use "flagpack-dart-sass/src/variables" with (
  $fp-prefix: "flag",
  $fp-countries: ("bo", "de", "us")
);
@use "flagpack-dart-sass/src/flagpack";

Since this changes the public configuration API for everyone using the Sass files directly, I'll release it as a major version bump.