Web-based block-editor

[gordonbrander]

This PR introduces a web-based block editor

• [x] Prosemirror based editor
  • [x] Prosmirror editor configuration for Subtext
  • [x] Editor web Component
  • [x] JS Subtext parser
  • [x] Inline subtext rendering using modeless markup
    • [x] Re-renders only changed blocks
  • [x] Transcludes beneath blocks
  • [x] Load transcludes from dummy data
  • [ ] Load transcludes from real data
• [x] SwiftUI wrapper for WKWebView
  • [ ] Modify inner scroll height of WKWebview to account for keyboard up
• [ ] Send data down to be displayed
• [ ] Send data up to be saved
• [ ] Markup toolbar shortcuts
• [ ] Link autocompletion