Add Sass::GemPackageImporter

[ntkme]

This is the counterpart of NodePackageImporter for loading pkg: URLs from RubyGems.

To gem maintainers: Because gemspec does not support declaring exports like package.json, you will have to put an _index.scss (or similar) at the root of the package if you want the users to be able to @use "pkg:gem_name";.

All the subpaths are looked up from the root of a gem. For example:

# frozen_string_literal: true

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'bootstrap', require: false
  gem 'sass-embedded'
end

puts Sass.compile_string('@use "pkg:bootstrap/assets/stylesheets/bootstrap";',
                         importers: [Sass::GemPackageImporter.new],
                         style: :compressed).css

[ntkme]

Normally we would have to decode the percent-encoding in the URL first, but in here we're lucky:

• The gem name part will only have "unreserved characters" in a URL and dart will unescape them as part of normalization if they are escaped. If we get a percent encoding in name it must be an invalid gem name, thus we don't have to worry.
• The subpath part we can just pass the escaped version back as part of the file URL as is.