feat: `st reorder` subcommand

[clabby]

Overview

Add a st reorder subcommand, which allows for reordering the branches within the current stack.

Functionality:

• Open an inquire::Editor prompt with the current stack loaded in, each on separate lines.
• Allow the user to adjust the order of the stack.
• When the user finishes, make the necessary modifications to the StackTree.
• Restack the current stack after the reorder.