Set `width` as aesthetic in `geom_col()`/`geom_bar()`.

[teunbrand]

This PR aims to fix #3142.

Motivation is given here: https://github.com/tidyverse/ggplot2/issues/3142#issuecomment-2019903367. To recap: yes variable width bars are bad, but we allowed them implicitly anyway and are jumping through hoops to clunkily discourage it. This PR removes the hack that recognises width as a parameter, and implements it as an aesthetic instead.