Implement `$dt$is_leap_year()` #765

Job	Run time
build (ubuntu-20.04, x86_64-unknown-linux-gnu)	18m 3s
build (ubuntu-20.04, aarch64-unknown-linux-gnu)	18m 56s
build (macos-14, x86_64-apple-darwin)	22m 57s
build (macos-14, aarch64-apple-darwin)	25m 42s
build (windows-latest, x86_64-pc-windows-gnu)	32m 10s
test (macos-12, release)	10m 47s
test (macos-14, release)	2m 57s
test (windows-latest, oldrel-1)	8m 1s
test (windows-latest, release)	4m 10s
test (ubuntu-latest, oldrel-1)	2m 13s
test (ubuntu-latest, release)	2m 6s
test (ubuntu-latest, devel)	7m 6s
merge	10s
release	0s
	2h 35m 18s

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