Use mementos for state management for pinned commands

Fixes #3977

The following changes are proposed:

Rather than updating user settings when commands are pinned/unpinned, we update a memento that tracks session to session state.
Add default settings for the cmake.pinnedCommands setting for the task runners.
Modify the behavior of the cmake.pinnedCommands setting. It no longer is a place that we store everything that is currently pinned. It only stores things that the user always wants to pin by default, regardless of whether they've unpinned it in the past.