Fix keyword as fmt argument

[dtolnay]

The standard library's format macros allow keyword named argument, as in format_args!("{type}", type = 1). These were previously crashing thiserror.

error: proc-macro derive panicked
   --> tests/test_display.rs:269:14
    |
269 |     #[derive(Error, Debug)]
    |              ^^^^^
    |
    = help: message: called `Result::unwrap()` on an `Err` value: Error("expected identifier")