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/EditorUpload.ts

@@ -1,5 +1,5 @@
 import {imageInfo} from '../../utils/image.ts';
-import {textareaInsertText, triggerEditorContentChanged} from './EditorMarkdown.ts';
+import {replaceTextareaSelection, triggerEditorContentChanged} from './EditorMarkdown.ts';
 import {
   DropzoneCustomEventRemovedFile,
   DropzoneCustomEventUploadDone,
@@ -43,7 +43,7 @@ class TextareaEditor {
   }
 
   insertPlaceholder(value: string) {
-    textareaInsertText(this.editor, value);
+    replaceTextareaSelection(this.editor, value);
   }
 
   replacePlaceholder(oldVal: string, newVal: string) {