Is your feature request related to a problem? Please describe.
No the current functionality is working as expected.

Describe the solution you'd like
I'd like to be able to toggle open/closed the bank & guild bank.
The current key bindings only allow us to open up the bank and guild bank. But if we press the keybind again, it doesn't close them, it simply ensures they are open.
I don't use the warband bank key binding, but others might want the same functionality for that key bind too. But if it's too much of a hassle, people could use the toggle bank key binding to close it since it uses the same window in the UI.

Describe alternatives you've considered
I have to manually close the window or press escape to close the windows, which can be annoying if I simply want to close one of the bank windows and keep everything else open.

Additional context
It appears the functionality to toggle the bank and guild bank already exists, as I can do it by clicking their respective buttons from the main bags UI.
I've also updated the Bindings_Mainline.xml file to trigger the BankToggle and GuildToggle events instead of the BankShow and GuildShow events, and everything works as expected.
This feature could be added as either new key bindings (toggle functionality), or by updating the existing key bindings with it being configurable in the addon options (open vs toggle functionality for key bindings).

Edit: I've created this PR to implement the above functionality, including switching between the character and warband tabs in the bank frame