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/SurfaceView_AppKit.swift

@@ -1041,7 +1041,8 @@ extension Ghostty.SurfaceView: NSServicesMenuRequestor {
     }
     
     func readSelection(from pboard: NSPasteboard) -> Bool {
-        guard let str = pboard.string(forType: .string) else { return false }
+        guard let str = pboard.getOpinionatedStringContents()
+        else { return false }
         
         let len = str.utf8CString.count
         if (len == 0) { return true }