Always resolve existing before showing new notification

[msujew]

What it does

Closes #13661

Using the vscode.window.show<Severity>Message API in VSCode behaves differently in Theia compared to VSCode when calling the method multiple times. Calling the method again with the same arguments will first off always resolve the previous call with undefined. Additionally, the newest message will always appear first in the toasts and notification center.

This change aligns Theia to this behavior.

How to test

1. Download and install this text extension (a modified notification extension sample).
2. Run the Show Info Notification and then the Show Warning Notification command. Both notifications should appear as expected
3. Run the Show Info Notification again. The initial call should resolve with undefined (this is indicated in the backend console). The notification should appear as the first notification in the toasts/notification center.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tortmayr]

Code looks good to me.
Tested with the provided extension and existing messages are now properly resolved before displaying a new notification. 👍🏼