Add CaptureService (#32)

https://devforum.roblox.com/t/captures-apis-are-now-available/2838188
https://create.roblox.com/docs/reference/engine/classes/CaptureService

index.d.ts

@@ -3,6 +3,7 @@ export declare const AnimationClipProvider: AnimationClipProvider;
 export declare const AssetService: AssetService;
 export declare const AvatarEditorService: AvatarEditorService;
 export declare const BadgeService: BadgeService;
+export declare const CaptureService: CaptureService;
 export declare const Chat: Chat;
 export declare const CollectionService: CollectionService;
 export declare const ContentProvider: ContentProvider;