Add a subseries field to card metadata (#305)

- Add an optional "subseries" field to the card metadata function
- If this is present, it overrides the main series field

#301

Reviewed-on: #305
Reviewed-by: VylpesTester <tester@vylpes.com>
Co-authored-by: Ethan Lane <ethan@vylpes.com>
Co-committed-by: Ethan Lane <ethan@vylpes.com>

src/contracts/SeriesMetadata.ts

@@ -11,6 +11,7 @@ export interface CardMetadata {
     name: string,
     type: CardRarity,
     path: string,
+    subseries?: string,
 }
 
 export interface DropResult {