Rename internal function set_linebreak_after_ggplot2_plus() to set_line_break_after_ggplot2_plus()

[Polkas]

closes #1048

Now all set_line_break_* functions have consistent naming.

As the set_linebreak_after_ggplot2_plus is not exported I think we do not have to add the record to the NEWS file i.e. add the */- to the commit message.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 53e52d8 is merged into main:

•   :ballot_box_with_check:cache_applying: 46ms -> 45.7ms [-7.97%, +6.54%]
•   :ballot_box_with_check:cache_recording: 1.47s -> 1.5s [-4.89%, +9.42%]
•   :ballot_box_with_check:without_cache: 3.88s -> 3.71s [-10%, +1.2%]

Further explanation regarding interpretation and methodology can be found in the documentation.

[IndrajeetPatil]

LGTM, thanks.