Improved codebase and fixes

Breaking Changes

• The public interface is redesigned. Please read example code in README to get familiar with the most important changes.

Fixes

• Fixed presentation of H6 element,
• Fixed rendering underline text that wasn't rendered correctly in some cases,
• Fixed rendering lines and image assets – several images are now rendered correctly when one is under another,
• Fixed rendering text when placed under image asset.

Added

• Dark mode support,
• When heading element is on a list, the bullet/number is rendered in the style of the heading,
• Code safety,
• Bumped Swift version,
• Updated documentation comments.