Add support for inline code info

[samuel-williams-shopify]

Summary

• update the cmark-gfm submodule and vendored sources to include inline code language prefixes
• expose code metadata through Node#code_info and add the Ruby-level Node#code_language convenience accessor
• render inline language classes through both native and Ruby HTML renderers
• document the opt-in parsing flag and add an Unreleased entry

Testing

• bundle exec bake test
• 2,209 tests, 2,344 assertions