Extra pointrange and errorbar support

[grantmcdermott]

Closes #38. Also closes #39.

Examples:

library(plot2)
par(pch = 19)

mod = lm(mpg ~ hp + factor(cyl), mtcars)
coefs = data.frame(names(coef(mod)), coef(mod), confint(mod))
coefs = setNames(coefs, c("x", "y", "ymin", "ymax"))

with(
  coefs,
  plot2(
    x = x,
    y = y,
    ymin = ymin,
    ymax = ymax,
    type = "pointrange"
  )
)

with(
  coefs,
  plot2(
    x = x,
    y = y,
    ymin = ymin,
    ymax = ymax,
    type = "errorbar"
  )
)

^{Created on 2023-06-19 with reprex v2.0.2}

[grantmcdermott]

I know everyone is busy with impending travels etc. and I'm pretty confident that this PR is good to go. So I'm happy to merge this as is, but please feel free to ping if you spot something problematic.