stat_summary_bin(width) of preferred over default width

[teunbrand]

This PR aims to fix #4647.

Briefly, it adds $compute_group(..., width), which is preferred over the computed bin width when provided by the user.

Reprex from issue:

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

ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, color = Species)) +
  stat_summary_bin(geom = "errorbar", width = 2)
#> No summary function supplied, defaulting to `mean_se()`
#> No summary function supplied, defaulting to `mean_se()`
#> No summary function supplied, defaulting to `mean_se()`

^{Created on 2024-07-03 with reprex v2.1.0}