Add a debug hook for getting sent unknown sequences from the XTerm pa…

…rser

This is here only for serving the keyboard checker/recorder tool.

src/textual/_xterm_parser.py

@@ -84,6 +84,13 @@ def parse_mouse_code(self, code: str) -> events.Event | None:
             return event
         return None
 
+    _reissued_sequence_debug_book: Callable[[str], None] | None = None
+    """INTERNAL USE ONLY!
+
+    If this property is set to a callable, it will be called *instead* of
+    the reissued sequence being emitted as key events.
+    """
+
     def parse(self, on_token: TokenCallback) -> Generator[Awaitable, str, None]:
         ESC = "\x1b"
         read1 = self.read1
@@ -94,6 +101,9 @@ def parse(self, on_token: TokenCallback) -> Generator[Awaitable, str, None]:
         use_prior_escape = False
 
         def reissue_sequence_as_keys(reissue_sequence: str) -> None:
+            if self._reissued_sequence_debug_book is not None:
+                self._reissued_sequence_debug_book(reissue_sequence)
+                return
             for character in reissue_sequence:
                 key_events = sequence_to_key_events(character)
                 for event in key_events: