Add pretty printer for &mut []

Fix #30232

Author: untitaker

src/etc/debugger_pretty_printers_common.py

@@ -139,7 +139,7 @@ def __classify_struct(self):
             return TYPE_KIND_STR_SLICE
 
         # REGULAR SLICE
-        if (unqualified_type_name.startswith("&[") and
+        if (unqualified_type_name.startswith(("&[", "&mut [")) and
             unqualified_type_name.endswith("]") and
             self.__conforms_to_field_layout(SLICE_FIELD_NAMES)):
             return TYPE_KIND_SLICE