Prototype for tab completion through handlers.
This provides a custom tab handler for DeclareDesign (see also ?completion and jimhester/completeme).
remotes::install_github("DeclareDesign/completion")
Each step declaration function in DeclareDesign has an internal handler function that is partially applied at the time of declaration.
For those functions, completion registers a completion function which can merge in the handler's arguments to the normal completions list.