Replace margin with gap for container components

[eduferfer]

Is your feature request related to a problem? Please describe.
Containers use spacing rules based on css margin which may conflict with margin rules of the slotted content itself.

Describe the solution you'd like
Instead, kor containers could be based on flex/grid gap rules, which are now supported by all modern browsers

• modularize container css props & add js doc
  • card (--body-gap, --header-gap, --functions-gap, --footer-gap)
  • app-bar (--functions-gap)
  • empty-state (--footer-gap)
  • notifications (--body-gap)
• bind encapsulated css props to inner modal & add js doc
  • accordion (--body-gap, --header-gap, --functions-gap, --footer-gap)
  • modal (--body-gap, --header-gap, --functions-gap, --footer-gap)
  • drawer (--body-gap, --header-gap, --functions-gap, --footer-gap)
  • popover (--body-gap, --header-gap, --functions-gap, --footer-gap)
  • tooltip (--body-gap, --header-gap, --functions-gap, --footer-gap)
  • notification-item (--body-gap, --header-gap, --functions-gap, --footer-gap)
  • pane (--body-gap, --header-gap, --functions-gap, --footer-gap)
• document
  • accordion
  • app-bar
  • card
  • drawer
  • empty-state
  • modal
  • notifications
  • notification-item
  • pane
  • popover
  • tooltip