[macOS] Updates Import/Export of OPML

- Created OPML UTType
- Added OPML as an importable UTI type
- `allowedFileTypes` replaced with `allowed ContentTypes`
- Moved UTType extension to single shared file
- Build flags updated

Shared/Extensions/UTType.swift

@@ -0,0 +1,19 @@
+//
+//  UTType.swift
+//  NetNewsWire
+//
+//  Created by Stuart Breckenridge on 31/05/2023.
+//  Copyright © 2023 Ranchero Software. All rights reserved.
+//
+
+import Foundation
+import UniformTypeIdentifiers
+
+public extension UTType {
+	static var nnwTheme: UTType {
+		UTType("com.ranchero.netnewswire.theme")!
+	}
+	static var opml: UTType {
+		UTType("public.opml")!
+	}
+}