T21a: Documents, editing

- markdown read with pulldown-cmark: marks for live styling, blocks, links, words
- forms::Highlight gains weight, slant and strike; measure and paint share runs
- Inter Italic embedded, so emphasis reads slanted
- forms::mention's popup and keys become Suggestions, shared with the editors
- RichTextEditor with FixedFormatToolbar, the floating toolbar, LinkEditor,
  @ and : suggestions, WordCount and ReadingTime
- MarkdownEditor: visual (a pressed block opens as source), source, split
- BlockEditor: prose, lists, to-dos, quotes, callouts, toggles, code, math,
  diagrams the app draws, dividers, tables that grow and merge, pictures to
  resize and align, cards for video and pages, columns, synced text; slash menu,
  markdown shortcuts, handles that drag and open a menu, one undo for all
- TodoItem and Checklist
- icons: heading-1..3, workflow, merge
- gallery: the Documents page with scripted states

src/lib.rs

@@ -4,6 +4,7 @@ pub mod buttons;
 pub mod charts;
 pub mod data_display;
 pub mod debug;
+pub mod documents;
 pub mod editor;
 pub mod feedback;
 pub mod finance;
@@ -44,4 +45,5 @@ pub fn init(cx: &mut App) {
     forms::bind_keys(cx);
     editor::bind_keys(cx);
     terminal::bind_keys(cx);
+    documents::bind_keys(cx);
 }