fix(Dropdown): default maxHeight with multiSelect/showSearch

[zettca]

• Fix HvDropdown's maxHeight when using with multiSelect & showSearch
  • moved to CSS vars in style, but kept the (hard-coded) height calculation logic
  • height passed to style as it's a user-defined prop
• Improve Docs/Test
  • merge samples related to height & search, and test it with Chromatic
  • inline Main story so changed args reflect on the source code (Storybook's generated code is good enough)
  • move Empty sample to Variants so it's also tested in Chromatic

Note

Also changes Visual Tests / Chromatic Checks to always ✅ in PRs.
Chromatic's own Checks will handle the rest

GitHub deployments are flaky again 🤷