Add new guide_circles()

[teunbrand]

This PR aims to fix #14.

devtools::load_all("~/packages/legendry/")
#> ℹ Loading legendry
#> Loading required package: ggplot2
library(gapminder)

df <- gapminder |>
  dplyr::filter(year == max(year))

ggplot(df, aes(gdpPercap, lifeExp, size = pop, fill = continent)) +
  geom_point(shape = 21) +
  scale_size_area(
    max_size = 20, 
    breaks = c(1e8, 4e8, 1e9), 
    labels = c("100M", "400M", "1B"),
    guide = guide_circles()
  )

^{Created on 2024-12-13 with reprex v2.1.1}