Command-line to toggle visibility

# Description of the new feature/enhancement

I'm proposing a command-line option to toggle the visibility of the terminal window. It should do exactly that: if I run (for instance) "wt --toggle" it should create a new window with default profile and show it (if no window exists) or show the current window (if one already exists). If the window is currently being displayed, running "wt --toggle" should hide the window and focus the previously selected application. This can be complemented with an option to make it that if "wt --toggle" creates the window, then it won't make it visible (useful on startup of terminal). A second run of "wt --toggle" will make the window visible.

# Proposed technical implementation details (optional)

Not sure about how to do this. Maybe a switch that will run the same "toggle visibility" internal command of terminal, or something? however that does not seem to be exactly what I described above.

No file or test is named. Start by tracing the command-line handling and the existing internal "toggle visibility" command mentioned in the issue, then determine how window creation, showing, hiding, and focus restoration are represented. Done means the command supports the described first-run, subsequent-toggle, and optional initially-hidden behaviors.

Written by the indexing model from the issue text.