Previous/Next Month Hotkeys for Calendar View

[leoccyao]

Is your feature request related to a problem? Please describe.
I'm able to use keyboard shortcuts to show the currently open daily note in the calendar, view, and to open the calendar view (#275). However, to navigate between months, I still have to use the mouse.

Describe the solution you'd like
Add a pair of hotkeys (can initially be unset) to navigate to the previous/next month in the calendar pane.

Describe alternatives you've considered
You could always open a daily note of a specific month then show it in the calendar, but this requires extra steps.

Additional context
Took a dig into the code, and displayedMonth is in the Calendar UI state and can be $set from outside the component, but I don't know Svelte and can't figure out if there's an equivalent way to get the state from outside the component.