Theming: Props Convention

[JasonGore]

• Style Variables vs. Component Variables vs. Props.
• Variants vs. Types.
  • Should variants be mutually exclusive?
  • Mutually exclusive "type" approach vs. Button experiments separated primary and circular props.
• What can be themed? All props or only style variables?
  • Fields limits theming to styles, theme and styleVariables currently.
  • Does any solution allow arbitrary props to be themed?
• Are style variables regular props or a special nested prop?
• Should we have common prop naming?
• Should themed props be merged with localized props?
• content prop vs. slots?
  • Is content always an implied slot?
  • Does the use of content props supercede a more explicitly named slot, such as label?

[JasonGore]

This issue and its questions have been resolved by #7012 and #5721.