fix: correctly type style override props

[AmauryLiet]

Current typing incorrectly enforces passing styling props as:

<Calendar
    theme={{
      stylesheet: {
        calendar: {
          header: {
            dayTextAtIndex0: { color: 'fuchsia', fontSize: 42 },
          }
        }
      }
    }}
/>

although this leads to ignored styling. Correct usage is:

<Calendar
    theme={{
      'stylesheet.calendar.header': {
        dayTextAtIndex0: { color: 'fuchsia', fontSize: 42 },
      }
    }}
/>

[SWARVY]

@AmauryLiet

Thank you so much for uploading this PR!

I'd like to share a suggestion regarding styling consistency that I've noticed while working with the library. Currently, there are two different approaches for styling calendar components in the original source code:

1. Other Custom style properties: Can be modified using stylesheet.~... approach

export default function getStyle(theme: Theme = {}) {
    ...other things...,
    ...(theme['stylesheet.calendar.main'] || {})
  });
}

2. ExpandableCalendar properties: Must use the currently defined types for styling to be applied

export default function styleConstructor(theme: Theme = {}) {
  const appStyle = {...defaultStyle, ...theme};

  return StyleSheet.create({
    ...other things...,
    ...(theme?.stylesheet?.expandable?.main || {})
  });
}

As a developer using this library, I've found this inconsistency to be a bit confusing when trying to understand which styling approach to use. It would be really helpful if we could have a more consistent styling pattern across all calendar components.

Suggestion: Would it be possible to consider standardizing the styling approach (either all stylesheet.~... or all current types) and applying it uniformly? This would make the library much easier to use and maintain for the community.