Contour breaks function works in absence of bins and binwidth

[teunbrand]

This PR aims to fix #5686.

Briefly, contour_breaks() will use a user-provided function even if bins and binwidth are NULL.
A missing binwidth argument is defaulted to 1/10^th the available range.

Reprex from #5686:

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

ggplot(faithfuld, aes(waiting, eruptions, z = density)) +
  geom_contour(
    breaks = \(x, y) seq(x[1], x[2], length.out = 4)
  )

^{Created on 2024-02-23 with reprex v2.1.0}

[thomasp85]

Are there other binning stats where the old behaviour is present? I think it is important that we ensure all behaves similarly

[teunbrand]

I don't think so because it appears that other binning stats don't accept functions for the breaks argument, see also #4561.