This was causing significant frame stutters for some players since the migration to .NET 5 in Stardew Valley 1.5.5.

This adds `#nullable disable` to all existing code (except where null is impossible like enum files), so it can be migrated incrementally.

This is incremented on each low-level tick (whether it's a game update, synchronized async operation, etc). That mainly avoids the cache persisting across the entire save loading process while it's being synchronized.

The game ticks aren't incremented consistently in some cases (e.g. while loading a save), which leads to the cache values being kept too long.

This is needed for the upcoming Stardew Valley 1.6 to avoid duplicate checks between DoesAssetExist and Load calls, and to make sure the answer doesn't change between them.