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/documents/mod.rs

@@ -0,0 +1,32 @@
+mod blocks;
+mod counts;
+mod format;
+mod link;
+mod markdown;
+mod modes;
+mod render;
+mod rich;
+mod suggest;
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
+mod todo;
+mod toolbar;
+mod tree;
+
+use gpui::App;
+
+pub use blocks::{Align, BlockData, BlockEditor, BlockEvent, BlockKind, Media};
+pub use counts::{ReadingTime, WordCount};
+pub use format::{Format, format};
+pub use link::LinkEditor;
+pub use markdown::markdown_highlights;
+pub use modes::{MarkdownEditor, MarkdownMode};
+pub use render::MarkdownRenderer;
+pub use rich::RichTextEditor;
+pub use todo::{Checklist, TodoItem};
+pub use toolbar::FixedFormatToolbar;
+
+/// Binds the documents keys. `init` calls it.
+pub(crate) fn bind_keys(cx: &mut App) {
+    rich::bind_keys(cx);
+}