Could `label_*` functions support labelling every nth break?

[davidhodge931]

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This is useful, as it provides another way to get the breaks & labels you want - and it can work well depending on the theme used.

I think it'd be ideal if this could be expressed as an argument every_nth argument in each label_* function.

This would then provide a way for the user to then determine the the frequency of labels (e.g. provide a label every 2nd break), and an offset (to start at not the first break if desired).

This is following on from this discussion here https://github.com/tidyverse/ggplot2/issues/5881