allow TRUE for as default for action = "flag"

[jmbarbone]

library(scribe)
ca <- command_args()
ca$add_argument("--foo", action = "flag", default = TRUE)
#> Warning: flag must be NULL or TRUE when action="flag"
ca$parse()
#> $foo
#> [1] FALSE
ca$set_input("--foo")
ca$parse()
#> $foo
#> [1] TRUE

^{Created on 2023-04-25 with reprex v2.0.2}