Seek better API to override components

[inokawa]

A few things we have to take into consideration:

• We may use virtua with some component libraries like Material UI, Radix UI, etc.
• Some elements require specific parent/children elements. For example ul requires li as children, and table requires hierarchy of table -> tbody -> tr -> td.
• In some case, we want to add padding to root component and it requires some special calculation. Paddings not taken into account in reverse mode  #216
• Adding header/footer/spinner to top/bottom of list is commonly used.

[inokawa]

https://material.angular.io/cdk/scrolling/overview#elements-with-parent-tag-requirements
https://material.angular.io/cdk/scrolling/overview#separate-viewport-and-scrolling-element

[inokawa]

https://github.com/microsoft/fluentui/tree/master/packages/react-components/react-virtualizer