feat(web): render editable tables and make images clickable

- Top-level GFM tables render as an editable widget: click a cell to
  edit its Markdown source, Tab/Enter/arrows to navigate, toolbar to
  insert/delete rows and columns or switch to source
- Structural table edits are isolated undo steps; focus is restored
  after the widget is rebuilt
- Clicking an image moves the cursor to its syntax to reveal the source
- Renderer escapes raw HTML and filters dangerous URL schemes by default

web/src/editor/livepreview.ts

@@ -138,7 +138,7 @@ function buildDecorations(view: EditorView): DecorationSet {
             classLines(node.from, node.to, () => 'cm-rm-code')
             return false
           case 'Table':
-            // 表格可视化编辑在 M2 实现，目前以等宽源码显示，内部语法不做隐藏以免列对不齐
+            // 光标在表格内时由此以等宽源码显示（其余情况由 table.ts 渲染为可编辑表格），内部语法不做隐藏以免列对不齐
             classLines(node.from, node.to, () => 'cm-rm-table')
             return false
           case 'HTMLBlock':