feat: add `excel` serializer/parser (#975) #1373

Job	Run time
R-CMD-check / setup	49s
routine / routine	3m 23s
website / pkgdown	3m 31s
R-CMD-check / macOS-latest (4.4.2)	2m 38s
R-CMD-check / windows-latest (4.4.2)	2m 42s
R-CMD-check / ubuntu-latest (devel)	3m 21s
R-CMD-check / ubuntu-latest (4.4.2)	3m 3s
R-CMD-check / ubuntu-latest (4.3.3)	6m 3s
R-CMD-check / ubuntu-latest (4.2.3)	4m 45s
R-CMD-check / ubuntu-latest (4.1.3)	3m 15s
R-CMD-check / ubuntu-latest (4.0.5)	3m 58s
	37m 28s

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