[VS Code Extension] Update Project Scaffolding, add options at the end of execution

[josefree]

Clear and concise description of the problem

Currently implementation is at the end of project scaffolding, it directly opens a new workspace with the project folder extended in the current window. Suggest to add an option as below.

Open is to open a new workspace in current window
Open in New Window is to open a new workspace in a new windows
Cancel is to keep in current workspace in current window <- this can meet the requirement of creating TypeSpec folder in an existing project.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[RodgeFu]

We will use vscode popup notification instead of modal dialog for user to choose the action as discussed. thanks.