Fix PowerShell window title + transient prompt

[kergoth]

The Starship support for a transient prompt requires that we call Enable-TransientPrompt. This calls Set-PSReadLineKeyHandler for the Enter key. Unfortunately, I like to set my window title based on the running command, which requires that I set the Enter key handler myself. I can't have a single handler that provides both pieces of functionality today, as Starship requires that a $script:TransientPrompt variable be set, but we can't set a $script variable in the starship module from outside the module. If starship provided a Set-TransientPrompt function to wrap the set of that script-internal variable, then we could copy the key handler and extend it.

• Submit an issue against the starship repository suggesting the addition of a Set-TransientPrompt function to allow for this.

This was submitted at starship/starship#6514, so this issue is currently blocked on that one.