Minor break function for log10 ticks

[teunbrand]

This PR aims to fix tidyverse/ggplot2#6016.

It adds a minor_breaks_log() function that mirrors the calculation in ggplot2::guide_axis_logticks().

By default, it tries to tone down the detailing as we have more powers of 10 covered.
In the example below, the y-axis has ~4 powers and every multiple of a power is marked.
However, in the x-axis spans ~7 powers and only every 5th multiple of a power, and the power itself is marked.

devtools::load_all("~/packages/scales")
#> ℹ Loading scales
library(ggplot2)

ggplot(msleep, aes(bodywt, brainwt)) +
  geom_point() +
  scale_x_log10(minor_breaks = minor_breaks_log()) +
  scale_y_log10(minor_breaks = minor_breaks_log())
#> Warning: Removed 27 rows containing missing values or values outside the scale range
#> (`geom_point()`).

^{Created on 2024-10-02 with reprex v2.1.1}

[thomasp85]

Thanks