[MINOR] Keyboard navigation (specifically zoom in/out)

[Risto-Stevcev]

Have you checked whether this feature is not already on the project board?

No

Is your feature request related to a problem? Please describe.
If you're on a keyboard mostly (or only) setup and you want to use org-roam-ui

Describe the solution you'd like
I don't have my trackpad enabled on purpose, I use the trackball instead, and so there's no way to zoom in/out either via keybindings or through the UI that I'm aware of. Zooming in/out with browser keys (Ctrl +/-) doesn't do what I'm expecting.

Describe alternatives you've considered
Using a mouse with a wheel scroll, but then I have to use a mouse.

Additional context
Add any other context or screenshots about the feature request here.