Add Typora extension syntax

- Parser extensions shared by the editor and the HTML renderer: ==highlight==, $inline math$
  (Pandoc rules, so amounts like $5 are not math), $$ math blocks, footnote references and
  definitions, [TOC], and YAML front matter
- Live preview: highlight, superscript/subscript, emoji names (gemoji), footnote references
  with the definition as tooltip, a clickable table of contents, styled front matter
- Math (KaTeX) and Mermaid render in place; while editing a block the source is shown with a
  live preview below. KaTeX, Mermaid and the emoji table are loaded only when a note uses them
- KaTeX is pinned to 0.18 so Mermaid shares the same copy
- Move editor focus tracking out of table.ts so block widgets and tables use the same state

web/src/editor/table.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import { createEditorState } from './index'
-import { setEditorFocus, tableField } from './table'
+import { setEditorFocus } from './focus'
+import { tableField } from './table'
 
 const doc = '| a | b |\n| --- | --- |\n| 1 | 2 |\n\n正文'