actually add PreviewCache oops

damus/Util/PreviewCache.swift

@@ -0,0 +1,35 @@
+//
+//  PreviewCache.swift
+//  damus
+//
+//  Created by William Casarin on 2023-01-02.
+//
+
+import Foundation
+import LinkPresentation
+
+enum Preview {
+    case value(LinkViewRepresentable)
+    case failed
+}
+
+class PreviewCache {
+    var previews: [String: Preview]
+    
+    func lookup(_ evid: String) -> Preview? {
+        return previews[evid]
+    }
+    
+    func store(evid: String, preview: LinkViewRepresentable?)  {
+        switch preview {
+        case .none:
+            previews[evid] = .failed
+        case .some(let meta):
+            previews[evid] = .value(meta)
+        }
+    }
+    
+    init() {
+        self.previews = [:]
+    }
+}