Optimize custom importer

[ntkme]

This PR substantially improves the performance of custom importers that return paths without source contents, by using FileImporter to load file from disk directly on dart-sass side, instead of reading from on ruby side and then passing to dart-sass via protobuf.

The current sass module resolution spec does not allow file extensions other than the standard ['.sass', '.scss', '.css']. The dart-sass implementation does not resolve if a custom FileImporter returns a url with non-standard extension. However, loading of sass files with non-standard extensions is used in sprockets. Therefore, in such case we have to fallback to an Importer to read the file on ruby side.

[ntkme]

require_relative 'lib/sassc-embedded'
require 'benchmark'

puts Sass.info

class CustomImporter < SassC::Importer
  def imports(path, _parent_path)
    Import.new(path)
  end
end

bootstrap = './vendor/github.com/twbs/bootstrap-rubygem/assets/stylesheets/_bootstrap.scss'

result = Benchmark.measure do
  100.times do
    SassC::Engine.new(File.read(bootstrap), { style: :compressed, filename: bootstrap, importer: CustomImporter }).render
  end
end

puts result

Before:

sass-embedded	1.72.0	(Embedded Host)	[Ruby]
dart-sass	1.72.0	(Sass Compiler)	[Dart]
  1.839168   0.735015   2.574183 ( 17.942375)

After:

sass-embedded	1.72.0	(Embedded Host)	[Ruby]
dart-sass	1.72.0	(Sass Compiler)	[Dart]
  0.967505   0.322428   1.289933 ( 16.500571)