Workspaces for handling multiple folders

[tomlin7]

Feature

Dev can be working with two folders in situations, and they might need to open both. So we need some sort of "workspaces" implementation.

• each new folder will be opened in a new window for the workspace
• when the workspace is closed, allow user to save the workspace state into a local file
• unlike vscode workspaces, the folders in a workspace need not share the same window or version control view or anything
• workspaces simply record the number of folders open into the state file, the state file can be used to restore the windows

NOTE

• add add folder to workspace, close workspace, save workspace commands
• prompt saving the workspace before closing it