feat(macOS): Paste copied files as absolute paths.

Previously files would be pasted as only the filename. This commit
introduces an extension to NSPasteboard which provides a method to
consistently get the string contents of a pasteboard so that the
behavior can stay the same anywhere where we need to do that.

macos/Sources/Ghostty/Ghostty.App.swift

@@ -382,7 +382,7 @@ extension Ghostty {
             }
             
             // Get our string
-            let str = NSPasteboard.general.string(forType: .string) ?? ""
+            let str = NSPasteboard.general.getOpinionatedStringContents() ?? ""
             completeClipboardRequest(surface, data: str, state: state)
         }