Add `var = ...` to `define_population` and `define_observation`

Add var = ... to define_population() and define_observation() in the vignette of https://merck.github.io/forestly/articles/forestly.html like code below.

adsl <- r2rtf::r2rtf_adsl |> 
  filter(TRT01P %in% c("Placebo", "Xanomeline High Dose")) |>
  mutate(TRT01P = factor(TRT01P,
                         levels = c("Placebo", "Xanomeline High Dose"),
                         labels = c("Control", "Experimental")),
         TRT01A = factor(TRT01A,
                         levels = c("Placebo", "Xanomeline High Dose"),
                         labels = c("Control", "Experimental")),
         RACE = stringr::str_to_sentence(RACE))

adae <- forestly_adae |> 
  filter(TRTA %in% c("Placebo", "Xanomeline High Dose")) |>
  rename(TRT01A = TRTA) |>
  mutate(TRT01A = factor(TRT01A,
                         levels = c("Placebo", "Xanomeline High Dose"),
                         labels = c("Control", "Experimental")))

meta_ae_forest <- meta_adam(population = adsl, 
                            observation = adae) |>
  define_plan(plan = plan(analysis = "ae_forestly",
                          population = "apat",
                          observation = "apat",
                          parameter = "any;drug-related;serious;drug-related-serious")) |>
  define_analysis(name = "ae_forestly", 
                  label = "Interactive Forest Plot") |>
  define_population(name = "apat", 
                    group = "TRT01A", 
                    id = "USUBJID",
                    subset = SAFFL == "Y", 
                    label = "All Patient as Treated",
                    var = c("USUBJID", "SAFFL", "TRT01A", "SITEID", "SEX", "RACE", "AGE")) |>
  define_observation(name = "apat", 
                     group = "TRT01A",
                     subset = SAFFL == "Y", 
                     label = "All Patient as Treated",
                     var = c("USUBJID", "SAFFL", "TRT01A", "AEDECOD", "AEBODSYS", 
                             "AEREL", "AESER", "AEOUT", "AEACN", "AESDTH", "ASTDT", "AENDT")
                     ) |>
  define_parameter(name = "any",
                   subset = NULL,
                   label = "Any AEs",
                   var = "AEDECOD", 
                   soc = "AEBODSYS") |>
  define_parameter(name = "drug-related",
                   subset = toupper(AEREL) %in% c("PROBABLE", "POSSIBLE"),
                   label = "Drug-related AEs",
                   var = "AEDECOD", soc = "AEBODSYS") |>
  define_parameter(name = "serious",
                   subset = AESER == "Y",
                   label = "Serious AEs",
                   var = "AEDECOD", soc = "AEBODSYS") |>
  define_parameter(name = "drug-related-serious",
                   subset = AESER == "Y" & toupper(AEREL) %in% c("PROBABLE", "POSSIBLE"),
                   label = "Drug-related serious AEs",
                   var = "AEDECOD", soc = "AEBODSYS") |>
  meta_build()

meta_ae_forest |>
  prepare_ae_forestly(ae_listing_display = c("USUBJID", "SITEID", "SEX", "RACE", "AGE",
                                             "AEREL", "AESER", "AEOUT", "AEACN", "AESDTH", "ASTDT", "AENDT")) |>
  format_ae_forestly(display = c("n", "prop", "fig_prop", "fig_diff")) |>
  ae_forestly()