feat: add prop to remove vertical margin between stacked items

[franciscohanna92]

closes #888

[lnagel]

This PR has been marked as outdated. Here's why:

This PR added a verticalMarginBetweenStackedItems boolean prop to control vertical spacing between stacked items. The codebase has since been rewritten from JavaScript to TypeScript (v0.30.0-beta.5), and this feature has been superseded by a more flexible approach: the new itemVerticalGap prop (a number, not a boolean) now provides precise control over the vertical gap between stacked items. It works alongside itemHeightRatio and takes precedence when specified. The stacking logic in src/lib/utility/calendar.tsx already uses itemVerticalGap throughout. Additionally, this PR targeted files that no longer exist in their original form — src/lib/Timeline.js is now src/lib/Timeline.tsx, tests have been migrated from Enzyme to @testing-library/react, and the __fixtures__ and __tests__ files referenced have been restructured.

Thank you for the contribution! The idea/fix was appreciated even if the codebase has moved on.