Add signed option for negative numbers in label_log()

[teunbrand]

This PR aims to fix #421.

Briefly, is adds a signed argument controlling wether + or - prefixes should be used.
By default, this is applied when negative numbers or zeroes are present in the numbers to label.

Reprex from the issue:

devtools::load_all("~/packages/scales/")
#> ℹ Loading scales

demo_continuous(
  c(-100, 100),
  transform = "asinh",
  breaks = c(-100, -10, -1, 0, 1, 10, 100),
  labels = label_log()
)
#> scale_x_continuous(transform = "asinh", breaks = c(-100, -10, 
#>     -1, 0, 1, 10, 100), labels = label_log())

^{Created on 2024-04-08 with reprex v2.1.0}

[teunbrand]

I took out the formatter for purposes of fixing #445.
The formatter works like this:

devtools::load_all("~/packages/scales/")
#> ℹ Loading scales
format_log(c(-10, -1, -0.1, 0, 0.1, 1, 10))
#> [1] "-10^ 1" "-10^ 0" "-10^-1" "0"      "+10^-1" "+10^ 0" "+10^ 1"

Which allows you to format geom_text(parse = TRUE):

library(ggplot2)
df <- data.frame(a = 1:5, b = 10^(1:5))
ggplot(df, aes(a, b)) +
  geom_point() +
  geom_text(
    aes(label = format_log(b)),
    parse = TRUE, vjust = -0.5
  )

^{Created on 2024-10-04 with reprex v2.1.1}

[thomasp85]

LGTM