Default types should be smarter about characters

[grantmcdermott]

For example, this should default to type_boxplot().

library(tinyplot)

set.seed(0)
x <- rep(c("A", "B", "C"), each = 20)
y <- runif(60)
gp <- sample(c("X", "Y", "Z"), size = 60, replace = TRUE)
df <- data.frame(x, y, gp)

plt(
  y ~ x | gp,
  data = df
)
#> Error in Summary.factor(structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, : 'range' not meaningful for factors

Separately, we shouldn't run into errors when x or y are characters/factors for points anyway. (This is the error above.) I'll address both issues in a PR shortly.

P.S. We coerce characters to factors lower in the main tinyplot.R code, but type sanitizing needs to happen earlier here.