Add matching pair insertion to markdown textarea (#36121)

1. Our textarea already has some editor-like feature like tab
indentation, so I thought why not also add insertion of matching closing
quotes/brackets over selected text. This does that.
2. `textareaInsertText` is replaced with `replaceTextareaSelection`
which does the same but create a new edit history entry in the textarea
so CTRL-Z works. The button that inserts tables into the textarea can
now also be reverted via CTRL-Z, which was not possible before.

web_src/js/features/comp/ComboMarkdownEditor.ts

@@ -17,7 +17,7 @@ import {POST} from '../../modules/fetch.ts';
 import {
   EventEditorContentChanged,
   initTextareaMarkdown,
-  textareaInsertText,
+  replaceTextareaSelection,
   triggerEditorContentChanged,
 } from './EditorMarkdown.ts';
 import {DropzoneCustomEventReloadFiles, initDropzone} from '../dropzone.ts';
@@ -273,7 +273,7 @@ export class ComboMarkdownEditor {
       let cols = parseInt(addTablePanel.querySelector<HTMLInputElement>('[name=cols]')!.value);
       rows = Math.max(1, Math.min(100, rows));
       cols = Math.max(1, Math.min(100, cols));
-      textareaInsertText(this.textarea, `\n${this.generateMarkdownTable(rows, cols)}\n\n`);
+      replaceTextareaSelection(this.textarea, `\n${this.generateMarkdownTable(rows, cols)}\n\n`);
       addTablePanelTippy.hide();
     });
   }