Add `leftMapOrKeepIn` / `leftFlatMapOrKeepIn` to `F[Either[A,B]]` syntax #843

Job	Run time
Generate Site (ubuntu-latest, temurin@17)	1m 15s
Build and Test (ubuntu-22.04, 2.12, temurin@8, rootJS)	2m 37s
Submit Dependencies	0s
Build and Test (ubuntu-22.04, 2.12, temurin@8, rootJVM)	2m 13s
Build and Test (ubuntu-22.04, 2.12, temurin@8, rootNative)	3m 2s
Build and Test (ubuntu-22.04, 3, temurin@8, rootJS)	2m 34s
Build and Test (ubuntu-22.04, 3, temurin@8, rootJVM)	2m 35s
Build and Test (ubuntu-22.04, 3, temurin@8, rootNative)	2m 47s
Build and Test (ubuntu-22.04, 2.13, temurin@8, rootJS)	2m 28s
Build and Test (ubuntu-22.04, 2.13, temurin@8, rootJVM)	2m 10s
Build and Test (ubuntu-22.04, 2.13, temurin@8, rootNative)	2m 40s
Build and Test (ubuntu-22.04, 2.13, temurin@17, rootJVM)	1m 22s
Publish Artifacts	0s
	25m 43s

Provide feedback