Drop support for sassc

[ntkme]

My proposal would be to produce a new MAJOR version of jekyll-sass-converter that:

1. Defaults to the sass-embedded implementation
2. Changes this import behavior
3. Removes the sassc compatibility entirely
4. Documents the upgrade path

Per above comment and discussion in #137, this PR drops support for sassc.

Changes

• Remove sassc.
• Bump required_ruby_version to >= 2.6.0 as it is a requirement for sass-embedded.
• Drop the undocumented add_charset option. Whether to add @charset or not will now be automatically determined based on associate_page_failed?. If a page is associated, sass will automatically add @charset or BOM if output contains any non-ASCII character. Otherwise, e.g. when called via sassify/scssify filters, @charset or BOM will be omitted.
• Generate embedded base64 source map for sassify/scssify filters if source map is enabled.
• Cherry-pick source map relative path change from Fix sassc relative import from input file #137.

[ashmaroli]

Thank you for getting the ball rolling towards a major version bump, @ntkme. I would like to see the Readme explicitly document the different out-of-the-box behaviors from v2.x and v3.x by the time this is ready for merge.
Once again, thanks for the huge effort put in already. 👍

[ntkme]

@ashmaroli @parkr Added a migration section in README and ready for initial review.

[ashmaroli]

@ntkme Is there a test for inline sourcemap with Base64 data..? I can't seem to find it.. if there is no test, it needs to be added as it is a new feature.

[ntkme]

@ashmaroli Finally got sometime to add tests around inline sourcemap.

[ashmaroli]

@ntkme I apologise for not telling you this earlier but I think you should extract the inline-sourcemap related changes to another branch and have it merged via a separate pull request that is meant to be merged at a future date.
Let this current pull request be just about removing legacy stuff and associated refactors.

[ntkme]

@ashmaroli Took it out.

[ashmaroli]

Is the following code meant to be in the current PR?

# Returns a source mapping url for given source-map.
def source_mapping_url(source_map)
  if associate_page_failed?
    "data:application/json;base64,#{Base64.strict_encode64(source_map)}"
  else
    Addressable::URI.encode(sass_page.basename + ".css.map")
  end
end

This looks like another enhancement to me..

[ntkme]

@ashmaroli Done.

[ashmaroli]

LGTM!

[ashmaroli]

Thanks @ntkme
@jekyllbot: merge +major