Build a better feature spotlight architecture

[justinrobots]

Description

We need a better system for tracking feature spotlights (popups, modals, etc.) that call out new or relevant features in the GitLens UI. We do this in various places across GitLens, but there isn't a consistent system for determining when these are displayed that ensures that they aren't displayed when they shouldn't be (e.g., if a message is intended to be seen once after a new feature is available, it shouldn't be shown again after being dismissed).