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 :

• Autoscroll file tree presenter when opened NewTools#699
• StFileOrFolder open should take the input as a possible selection NewTools#808
• Add indexOf: to the List Presenter #1585
• Scrolling should scroll both lists hernanmd/class-diff#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: