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/expression.rs

@@ -237,13 +237,7 @@ pub fn expression_highlights(source: &str, cx: &App) -> Vec<(Range<usize>, Highl
                 Token::Op(_) => syntax.operator,
                 Token::Open | Token::Close | Token::Comma => syntax.punctuation,
             };
-            (
-                range.clone(),
-                Highlight {
-                    color,
-                    background: None,
-                },
-            )
+            (range.clone(), Highlight::new(color))
         })
         .collect()
 }