Add raw value to `unknown` enum cases

[JacobHearst]

Motivation

As a developer using ScryfallKit, I would like the unknown enum cases in the library to expose the enum case that couldn't be decoded so that I can display the raw value instead of "unknown" in my app.

Acceptance Criteria

• Card.Layout.unknown, Card.FrameEffect.unknown, and MTGSet.Type hold onto the raw value that couldn't be decoded to a known value
• unknown cases have documentation explaining their purpose and use

[JacobHearst]

@missingems anything else you want to add to this? This is what I've done so far

[missingems]

looks good to me