Add centerProminently command

[vincentpun]

Changes:

• Add a CenterProminently command that centers the window horizontally, but moves the window up vertically slightly from the center to give visual importance.
• Add documentation to TerminalCommands.md

This follows from the discussion here https://github.com/rxhanson/Rectangle/discussions/1285. Thank you for making the code easy to follow!

[rxhanson]

Looks good! Thanks!