Add Keyword Rest design proposal

[AlexKnauth]

Adds a markdown document with a design proposal for & rest arguments and ~& keyword rest arguments, as well as their potential interaction with []/{} data constructors and ... ellipses.

Preview: https://github.com/AlexKnauth/rhombus-prototype/blob/keyword-rest-design/design/keyword-rest/keyword-rest.md

See also https://github.com/racket/rhombus-prototype/discussions/219 and https://github.com/racket/rhombus-prototype/issues/66.