`parse_formula()` is not robust to variable names with a `-` sign

[Christophe-Regouby]

Current behavior

Depending of the variable name characters, plotluck() may wrongly interpret formula as invalid. a variable name with the minus sign will make plotluck fails with

Error in parse.formula(formula) : 
  Invalid formula: only one dependent variable allowed

Expected behavior

Allow variable names with - sign

ReprEx

library(plotluck)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(gapminder)
gapminder <- gapminder |> rename(`int-year` = "year")
plotluck(gapminder, .~1)
#> Factor variable country has too many levels (142), truncating to 30
#> Error in parse.formula(formula): Invalid formula: only one dependent variable allowed

^{Created on 2025-01-29 with reprex v2.1.1}