GitHub Flavored Markdown and native SwiftUI rendering

[gonzalezreal]

About

This pull request contains a rewrite of the library that brings the following features:

• GitHub Flavored Markdown (autolinks, strikethrough text, task lists, and tables)
• Native SwiftUI rendering
• Themes and full customization of inlines and blocks

Breaking changes

• MarkdownUI now requires macOS 12+, iOS 15+, tvOS 15+ and watchOS 8+
• MarkdownUI now uses SwiftUI primitives to render Markdown and no longer depends on gonzalezreal/AttributedText.
• MarkdownUI brings a new DSL to create Markdown content and no longer depends on gonzalezreal/SwiftCommonMark.
• MarkdownStyle and all of its subtypes are no longer available. Use the new Theme API to customize the Markdown appearance.
• MarkdownImageHandler is no longer available. Use the new ImageProvider API to customize the image loading behavior.

Status

Markdown rendering and styling:

• [x] Inline text
• [x] Images
• [x] Paragraphs
• [x] Blockquotes
• [x] Task lists
• [x] Lists
• [x] Code blocks
• [x] Tables
• [x] Headings
• [x] Thematic breaks

Other stuff:

• [x] Theme support
• [x] DocC theme
• [x] GitHub theme
• [x] Sample code
• [x] watchOS support
• [x] Heading anchors
• [x] API to integrate a code syntax highlighter
• [x] API to integrate other image loading systems
• [x] Documentation

[nicksiscoe]

Just droppin' in to provide moral support - This will be a game-changing PR ;)