Feat: Add preview as plaintext toggle for Preview As Text (#7102)

## Summary

Adds a toggle to conditionally render the text as Markdown.

## Also...

Fixes some type issues across our myriad Widget types. We should
probably clean those up.

## Example


https://github.com/user-attachments/assets/24fed943-1e79-4ea4-a962-826b06d68761



This could be a good minimal testcase for dynamic widgets @AustinMroz

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7102-WIP-Feat-Add-preview-as-plaintext-toggle-for-Preview-As-Text-2bd6d73d3650810c8b25c84866c8875c)
by [Unito](https://www.unito.io)

src/extensions/core/previewAny.ts

@@ -24,12 +24,43 @@ useExtensionService().registerExtension({
           app
         ).widget as DOMWidget<HTMLTextAreaElement, string>
 
-        showValueWidget.options.read_only = true
+        const showValueWidgetPlain = ComfyWidgets['STRING'](
+          this,
+          'preview',
+          ['STRING', { multiline: true }],
+          app
+        ).widget as DOMWidget<HTMLTextAreaElement, string>
 
+        const showAsPlaintextWidget = ComfyWidgets['BOOLEAN'](
+          this,
+          'previewMode',
+          [
+            'BOOLEAN',
+            { label_on: 'Markdown', label_off: 'Plaintext', default: false }
+          ],
+          app
+        )
+
+        showAsPlaintextWidget.widget.callback = (value) => {
+          showValueWidget.hidden = !value
+          showValueWidget.options.hidden = !value
+          showValueWidgetPlain.hidden = value
+          showValueWidgetPlain.options.hidden = value
+        }
+
+        showValueWidget.hidden = true
+        showValueWidget.options.hidden = true
+        showValueWidget.options.read_only = true
         showValueWidget.element.readOnly = true
         showValueWidget.element.disabled = true
-
         showValueWidget.serialize = false
+
+        showValueWidgetPlain.hidden = false
+        showValueWidgetPlain.options.hidden = false
+        showValueWidgetPlain.options.read_only = true
+        showValueWidgetPlain.element.readOnly = true
+        showValueWidgetPlain.element.disabled = true
+        showValueWidgetPlain.serialize = false
       }
 
       const onExecuted = nodeType.prototype.onExecuted
@@ -39,9 +70,10 @@ useExtensionService().registerExtension({
           ? void 0
           : onExecuted.apply(this, [message])
 
-        const previewWidget = this.widgets?.find((w) => w.name === 'preview')
+        const previewWidgets =
+          this.widgets?.filter((w) => w.name === 'preview') ?? []
 
-        if (previewWidget) {
+        for (const previewWidget of previewWidgets) {
           previewWidget.value = message.text[0]
         }
       }