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/package.json

@@ -17,6 +17,9 @@
     "@lezer/common": "^1.5.3",
     "@lezer/highlight": "^1.2.5",
     "@lezer/markdown": "^1.8.0",
+    "gemoji": "^8.1.0",
+    "katex": "^0.18.11",
+    "mermaid": "^12.2.0",
     "shiki": "^4.5.0",
     "vue": "^3.5.44"
   },