feat: create "Add package" EditorNotificationPanel if derive_targets_from_directories is on

[tpasternak]

Based on #6359 by @dkashyn-sfdc

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Description of this change

[tpasternak]

The code contains some code duplications, but otherwise it would require either to put some if statements in the middle of the bodies, or to rework the whole file, which we would like to avoid @ujohnny @LeFrosch

[ujohnny]

So initially there was a notification

Do you want to add this file to your project sources?           Add file to project

And with this change it in some cases looks like

Do you want to add this file to your project sources?           Add package to project

I'd consider updating notification text for the new action into something like

Do you want to add this package to your project sources?  Add package to project

Also I checked the code on a basic example with directories: . and click on the notification does not do anything. I guess this has to be checked before displaying the notification

Screencast.from.2024-04-18.16-33-45.webm

[ujohnny]

This is always null

[ujohnny]

It seems to me that some regions have formatting issues. I believe the indent should be 4 spaces instead of 8.

[ujohnny]

lgtm, but agreed to extract duplicated constant to variable in a separate PR