Add scroll to index to Lists

[hernanmd]

This PR adds a method, scrollToIndex:, to support vertical list scrolling in Spec. Currently working in the Morphic backend. Its functionality is necessary to fix multiple issues, such as :

• https://github.com/pharo-spec/NewTools/issues/699
• https://github.com/pharo-spec/NewTools/issues/808
• https://github.com/pharo-spec/Spec/issues/1585
• https://github.com/hernanmd/class-diff/issues/1 and probably others

It also adds corresponding tests.

[Ducasse]

Cyril this is strange since there are reference to Author in the test log

MetacelloNotification: Loaded -> SmalltalkCI-Pharo-Coverage-Core --- filetree:///home/runner/.smalltalkCI/repository [:]()
MetacelloNotification: ...finished baseline
KeyNotFound: key #Author not found in SystemEnvironment
SystemEnvironment(Dictionary)>>errorKeyNotFound:
[self errorKeyNotFound: key] in SystemEnvironment(Dictionary)>>at: in Block: [self errorKeyNotFound: key]
SystemEnvironment(Dictionary)>>at:ifAbsent:
SystemEnvironment(Dictionary)>>at: