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/lib/litegraph/src/LGraphNode.ts

@@ -4027,7 +4027,9 @@ export class LGraphNode
       w: IBaseWidget
     }[] = []
 
-    for (const w of this.widgets) {
+    const visibleWidgets = this.widgets.filter((w) => !w.hidden)
+
+    for (const w of visibleWidgets) {
       if (w.computeSize) {
         const height = w.computeSize()[1] + 4
         w.computedHeight = height
@@ -4066,7 +4068,7 @@ export class LGraphNode
 
     // Position widgets
     let y = startY
-    for (const w of this.widgets) {
+    for (const w of visibleWidgets) {
       w.y = y
       y += w.computedHeight ?? 0
     }