Stage Manager: Offer a 90% Snap Option

Hello thank you for updating BetterSnapTool to work with Stage Manager!

Something I'm currently struggling with is I'd like to be able to easily snap between full screen and the 90% snap option defined in the screenshot below i.e. leaving a gap to the left for stage manager stages.

Essentially it'd be great if it was possible to add the 90% snap option as one of the keyboard shortcuts.

The use case here is being able to jump between having a window open to see stages on the left versus going full screen when desired. Thanks!