Rename `group`/`group_by` arguments into `by` #469

Job	Run time
R-CMD-check / macOS-latest (release)	6m 11s
R-CMD-check / windows-latest (devel)	18m 57s
R-CMD-check / windows-latest (release)	19m 41s
R-CMD-check / windows-latest (oldrel)	19m 27s
R-CMD-check / windows-latest (4.1)	4m 46s
R-CMD-check / ubuntu-latest (devel)	14m 10s
R-CMD-check / ubuntu-latest (release)	9m 5s
R-CMD-check / ubuntu-latest (4.3)	25m 38s
R-CMD-check / ubuntu-latest (4.2)	1m 3s
R-CMD-check / ubuntu-latest (4.1)	1m 4s
R-CMD-check / ubuntu-latest (4.0)	1m 5s
R-CMD-check / ubuntu-latest (3.6)	48s
	2h 1m 55s

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