refactor(BlockEditor): replace emoji icons with react icon components and add free color picker support

- update blockRegistry to accept ReactNode icons instead of emoji strings
- replace emoji icons in all built-in block types with icon components from shared icons
- add `isHexColor` and `collectUsedColors` helpers to inline/types.js
- extend `color` and `highlight` marks to accept hex color strings in addition to palette keys
- pass `usedColors` (collected from document) to InlineToolbar
- update InlineToolbar color popover to show used colors and a free color input
- add new icons to shared icons index
- update README to reflect icon, color, and toolbar popover changes

src/shared/components/BlockEditor/index.js

@@ -22,6 +22,8 @@ export { newBlockId } from './utils/ids.js';
 export {
   INLINE_COLORS,
   INLINE_COLOR_KEYS,
+  isHexColor,
+  collectUsedColors,
   inlineLength,
   inlineToPlainText,
   inlineFromText,