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ARROW-13562: [R] Styler followups

Adds styling tasks to the Makefile (for 🦖  like me; I found that the styling-on-save from vscode was not reliable). Also makes codegen.R generate styled R code.

Closes #10879 from nealrichardson/styler2

Lead-authored-by: Jonathan Keane <jkeane@gmail.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>