UI Builder does not warn users or ask for confirmation before a new file is opened/created when there are no unsaved changes

Steps to reproduce:

1. Open a Unity project
2. Open the UI Builder via Window > UI Toolkit > UI Builder
3. Make some changes to the unsaved file and then save with any name via File > Save on the toolbar
4. Create a new file within UI Builder via File > New on the toolbar

Actual results: A new file is created and shown within the UI Builder window. The previous file is no longer shown within the window unless it is re-opened.

Expected results: As the previous file will be closed and a new one opened, it is expected that the user will be asked for confirmation (or at least warned).

Reproducible with versions: 

• 6000.0.13f1
• 2022.3.40f1
• 2021.3.40f1

Not reproducible with versions: - 

Can’t test with versions: - 

Tested on (OS): Windows 10, Windows 11

Notes:

• This is a usability issue because UI Builder is a monolithic window and doesn't support multi-windows. While other areas of the Unity Editor (and other applications like Google Docs, e.t.c) do not usually ask for confirmation (or give a warning) when opening or creating a new file, it does not affect usability in those cases as it opens a new window and the previous file is not closed. Within UI Builder, this is not the case and the previous file has to be closed for a new one to be loaded in.