debug: improve debug configuration behavior for multi-root workspaces

[Maddobun]

What it does

This PR fixes #12663 and introduces improvements to the theia debug widget:

• debug configuration dropdown now listen to changes in workspace root. If a root is added or removed for a multi-root project, the debug widget will refresh to reflect the change.
• the open launch.json button now opens the configuration selected in the drop down menu instead of the first root folder configuration.

How to test

• Open a multi-root work space. Verify that launch configuration for each root show up in the debug dropdown
• Remove one of the root folder by right click -> remove folder from workspace
• Verify that the associated launch configuration disappears
• Verify that open launch.json button opens the configuration selected in the drop down menu

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

[vince-fugnitto]

Thank you for your contribution 👍

In order to accept your changes please be sure to sure to sign the eclipse contributor agreement (eca) with the same email as your authorship.

[vince-fugnitto]

LGTM 👍

I confirmed that:

• adding/removing roots properly updates the debug configurations
• given the selected configuration, open configuration properly opens the launch.json of the appropriate root