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/primitives/icon.rs

@@ -146,6 +146,9 @@ icons! {
     HardDrive => "hard-drive",
     Hash => "hash",
     Heading => "heading",
+    Heading1 => "heading-1",
+    Heading2 => "heading-2",
+    Heading3 => "heading-3",
     Heart => "heart",
     History => "history",
     Hourglass => "hourglass",
@@ -180,6 +183,7 @@ icons! {
     Maximize2 => "maximize-2",
     MemoryStick => "memory-stick",
     Menu => "menu",
+    Merge => "merge",
     MessageSquare => "message-square",
     MicOff => "mic-off",
     Mic => "mic",
@@ -272,6 +276,7 @@ icons! {
     WholeWord => "whole-word",
     WifiOff => "wifi-off",
     Wifi => "wifi",
+    Workflow => "workflow",
     Wrench => "wrench",
     X => "x",
     Zap => "zap",