fix(init/pwsh): preserve ViModeChangeHandler

[HeyItsGilbert]

If a user has defined a ViModeChangeHandler setting this would respect that as well as redraw the prompt (to allow rendering things like the character change).

Description

Users don't realize that the ViModeChangeHandler is redrawing the prompt. If the user sets their own, as show in the PSReadLine example to change cursor type, they don't realize it'll be overwritten by the starship init powershell call.

Motivation and Context

Closes #6217

Screenshots (if appropriate):

How Has This Been Tested?

• [ ] I have tested using MacOS
• [ ] I have tested using Linux
• [x] I have tested using Windows

Checklist:

• [ ] I have updated the documentation accordingly.
• [ ] I have updated the tests accordingly.