Fix backspace echo for terminals that send chr(127)

The input detection handled both chr(8) and chr(127) but the echo
logic only checked chr(8). Most modern terminals send chr(127) for
backspace, so the cursor never moved back visually.

src/mudlib/zmachine/trivialzui.py

@@ -368,7 +368,7 @@ def _read_line(original_text=None, terminating_characters=None):
 
         if char == "\r":
             char_to_print = "\n"
-        elif char == _BACKSPACE_CHAR:
+        elif char in (_BACKSPACE_CHAR, _DELETE_CHAR):
             char_to_print = f"{_BACKSPACE_CHAR} {_BACKSPACE_CHAR}"
         else:
             char_to_print = char