Shell: Add support for escaping backslash in prompt sequence

Author: https://github.com/adamjoer
Commit: https://github.com/SerenityOS/serenity/commit/491d61a2fd
Pull-request: https://github.com/SerenityOS/serenity/pull/21589
Reviewed-by: https://github.com/alimpfard

Userland/Shell/Shell.cpp

@@ -157,6 +157,9 @@ DeprecatedString Shell::prompt() const
             else
                 builder.append('!');
 
+        } else if (lexer.consume_specific('\\')) {
+            builder.append('\\');
+
         } else {
             lexer.consume();
         }