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/highlight.rs

@@ -0,0 +1,24 @@
+use gpui::{FontWeight, Hsla};
+
+/// Styles a span of the text: its color, an optional wash behind it, and its weight, slant and strike.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct Highlight {
+    pub color: Hsla,
+    pub background: Option<Hsla>,
+    pub weight: Option<FontWeight>,
+    pub italic: bool,
+    pub strike: bool,
+}
+
+impl Highlight {
+    /// A span in `color`, its font as the field's.
+    pub fn new(color: Hsla) -> Self {
+        Self {
+            color,
+            background: None,
+            weight: None,
+            italic: false,
+            strike: false,
+        }
+    }
+}