Create helper extensions for Block and update tests for the Block helper model

Closes: https://github.com/damus-io/damus/pull/1528
Signed-off-by: William Casarin <jb55@jb55.com>

damusTests/Models/Block+Tests.swift

@@ -0,0 +1,56 @@
+//
+//  Block+Tests.swift
+//  damusTests
+//
+//  Created by Kyle Roucis on 9/1/23.
+//
+
+import Foundation
+@testable import damus
+
+
+extension Block {
+    var asText: String? {
+        switch self {
+        case .text(let text):
+            return text
+        default:
+            return nil
+        }
+    }
+    
+    var isText: Bool {
+        return self.asText != nil
+    }
+    
+    var asURL: URL? {
+        switch self {
+        case .url(let url):
+            return url
+        default:
+            return nil
+        }
+    }
+    
+    var isURL: Bool {
+        return self.asURL != nil
+    }
+    
+    var asMention: Mention<MentionRef>? {
+        switch self {
+        case .mention(let mention):
+            return mention
+        default:
+            return nil
+        }
+    }
+    
+    var asHashtag: String? {
+        switch self {
+        case .hashtag(let hashtag):
+            return hashtag
+        default:
+            return nil
+        }
+    }
+}