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/text/mod.rs

@@ -1,18 +1,20 @@
 mod actions;
 mod edit;
 mod element;
+mod highlight;
 mod ime;
 
 use std::{ops::Range, rc::Rc, time::Duration};
 
 use gpui::{
-    App, Bounds, Context, EventEmitter, FocusHandle, Focusable, Hsla, Pixels, Point, SharedString,
+    App, Bounds, Context, EventEmitter, FocusHandle, Focusable, Pixels, Point, SharedString,
     Subscription, Task, Window, WrappedLine,
 };
 
-pub(crate) use actions::{Backspace, Down, Enter, Submit, Up, bind_keys};
+pub(crate) use actions::{Backspace, Down, Enter, Redo, Submit, Undo, Up, bind_keys};
 use edit::Snapshot;
 pub(crate) use edit::{History, from_utf16, to_utf16};
+pub use highlight::Highlight;
 
 use crate::theme::ActiveTheme;
 
@@ -26,13 +28,6 @@ pub enum InputEvent {
     Blur,
 }
 
-/// Colors a span of the text: foreground, and an optional wash behind it.
-#[derive(Clone, Copy, Debug, PartialEq)]
-pub struct Highlight {
-    pub color: Hsla,
-    pub background: Option<Hsla>,
-}
-
 type Highlighter = Rc<dyn Fn(&str, &App) -> Vec<(Range<usize>, Highlight)>>;
 type Filter = Rc<dyn Fn(char) -> bool>;
 pub(crate) type Fit = Rc<dyn Fn(&str, Range<usize>, &str) -> (String, usize)>;