Added the catalogue models and seed to support phase 5 of the POC prototype in the Persistence package.

Packages/Persistence/Sources/Public/Models/Listing.swift

@@ -152,6 +152,24 @@ public extension Listing {
         keyNotes != nil
     }
 
+    /// The issue's number within its series, or `nil` for a copy that is not a numbered single issue.
+    var issueNumber: Int? {
+        format == "single-issues"
+            ? Int(issueLabel.drop { !$0.isNumber }.prefix { $0.isNumber })
+            : nil
+    }
+
+    /// The name of the series the copy belongs to: its series title, less the `(2019 series)` a single issue's carries.
+    var seriesName: String {
+        seriesTitle
+            .replacing(#/\s\(\d{4} series\)$/#, with: "")
+    }
+
+    /// The slug of the series the copy belongs to.
+    var seriesSlug: String {
+        ListingDraft.slug(seriesName)
+    }
+
     /// Whether the copy is graded and slabbed by a grading company.
     var isSlabbed: Bool {
         grader != nil