reprex_alert() errors with its default type argument

[sims1253]

Found this during my most recent ry audit

Summary

The internal reprex_alert() helper errors at runtime whenever it is called
with its declared default type argument, because the default is passed
straight to switch() without first reducing it to a scalar.

Reproducible example

pkgload::load_all()
local_reprex_loud()  # or run outside of testing
reprex:::reprex_alert("hello")
#> Error in switch(type, success = cli::cli_alert_success, info = cli::cli_alert_info,  :
#>   EXPR must be a length 1 vector

Root cause

reprex_alert() declares the match.arg()-style default but never calls
match.arg():

reprex_alert <- function(
  text,
  type = c("success", "info", "warning", "danger"),
  .envir = parent.frame()
) {
  quiet <- reprex_quiet() %|% is_testing()
  if (quiet) {
    return(invisible())
  }
  cli_fun <- switch(           # <-- `type` is length 4 here
    type,
    success = cli::cli_alert_success,
    ...
  )
}

With the default, type is the length-4 character vector
c("success", "info", "warning", "danger"), and switch() requires a
length-1 EXPR, so it errors with EXPR must be a length 1 vector.