Add Markdown Preview Toggle (#15215)

Add a "Preview Markdown" button to the quick action bar when in a
markdown editor.

While it isn't my favorite, I went with the basic eye icon to be a bit
more generic so we can extend this control to allow opening other
previews such as SVGs like @jansol mentioned.

![CleanShot 2024-07-26 at 11 02
16@2x](https://github.com/user-attachments/assets/415963ce-d19e-432d-b8c2-37e7c6e52683)


https://github.com/user-attachments/assets/5980272c-eab9-4f69-86b6-0c593c25b525

---

Release Notes:

- Added a button to preview Markdown files in the toolbar.
`Option|Alt+Click` will open the preview to the side.

assets/icons/eye.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-eye"><path d="M2.062 12.348a1 1 0 0 1 0-.696 10.75 10.75 0 0 1 19.876 0 1 1 0 0 1 0 .696 10.75 10.75 0 0 1-19.876 0"/><circle cx="12" cy="12" r="3"/></svg>

assets/icons/file_code.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-file-code"><path d="M10 12.5 8 15l2 2.5"/><path d="m14 12.5 2 2.5-2 2.5"/><path d="M14 2v4a2 2 0 0 0 2 2h4"/><path d="M15 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V7z"/></svg>

assets/icons/file_text.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-file-text"><path d="M15 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V7Z"/><path d="M14 2v4a2 2 0 0 0 2 2h4"/><path d="M10 9H8"/><path d="M16 13H8"/><path d="M16 17H8"/></svg>