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/forms/mod.rs

@@ -64,6 +64,7 @@ pub use date::{
 };
 pub use expression::{ExpressionInput, evaluate, expression_highlights};
 pub use files::{DropZone, FileInput};
+pub(crate) use glyphs::emoji_found;
 pub use glyphs::{EmojiPicker, IconPicker};
 pub use group::{InputAddon, InputGroup};
 pub use hotkey::HotkeyInput;
@@ -74,6 +75,7 @@ pub use knob::Knob;
 pub use listbox::ListBox;
 pub use masked::{COUNTRIES, MaskedInput, PhoneInput};
 pub use mention::{MentionInput, mention_highlights};
+pub(crate) use mention::{Suggestions, active_trigger, handles_matching};
 pub use multi::MultiSelect;
 pub use number::{NumberInput, ScrubInput};
 pub use options::Choice;
@@ -95,7 +97,9 @@ pub use structure::{
 };
 pub use switch::Switch;
 pub use tags::TagInput;
-pub(crate) use text::{Down, Enter, History, Submit, Up, bind_keys, from_utf16, to_utf16};
+pub(crate) use text::{
+    Backspace, Down, Enter, History, Redo, Submit, Undo, Up, bind_keys, from_utf16, to_utf16,
+};
 pub use text::{Highlight, InputEvent, TextInput};
 pub use transfer::TransferList;
 pub use unit::UnitInput;