fix: keyboard focus does not return to previous element after closing FluentDialog

[adamint]

🐛 Bug Report

As in title, keyboard focus should be placed back on the element focused before the dialog was opened after it has been closed. This is apparent when triggering dialog open using the keyboard (with enter).

💻 Repro or Code Sample

Can be reproduced in any of the FluentDialog examples -> https://www.fluentui-blazor.net/Dialog

💁 Possible Solution

Save a reference to the element currently focused during dialog open, in the dialog service. Focus it again during close.

I will follow up with a fix tomorrow.

🔦 Context

microsoft/aspire#10119

[vnbaaij]

Fixed with PR #4095