Replace KeyValue implementation with ValveKeyValue library

[xPaw]

Depends on ValveResourceFormat/ValveKeyValue#30

Would have to figure out how to do it in a way to reduce breaking changes.

Fixes #258

[yaakov-h]

We could probably write a wrapper that keeps the old KeyValue API and flag it with a warning-class [Obsolete] attribute, to give consumers some time to migrate?

Do we need to keep binary compatibility, source compatibility, or neither-but-make-it-easy?

[xPaw]

Binary compat sounds like a bad idea considering we're going for .net 8 anyway.

And some SK apis use KeyValues so I'm not sure how obsolete helps if you can't migrate.