Fix #5284: Restore worksheet decorations
Fix #5284: Restore worksheet decorations

When switching tabs, VSCode discards the text decorations that were
added by worksheet evaluation. We now keep them in memory so that they
can be added again when coming back to the worksheet.

lampepfl/dotty,lampepfl/dotty,som-snytt/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty