Add with to PlainYearMonth #753

Job	Run time
Test feature matrix	1m 58s
Documentation	17s
Test (stable, ubuntu-latest)	17s
Test FFI (stable, ubuntu-latest)	11s
Test (stable, macos-latest)	24s
Test (stable, windows-latest)	1m 46s
Lint	13s
MSRV	20s
	5m 26s

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