message: Use split1_opt from gazebo in format impl
  Summary: Follow-up to (https://github.com/facebookexperimental/starlark-rust/commit/e47bcb526667b2c77b1090e02d77e29759439f1d).
diff: @@ -429,8 +429,8 @@ fn format_capture<'v, T: Iterator<Item = Value<'v>>>(
  result: &mut String,
  ) -> anyhow::Result<()> {
  let (n, conv) = {
  - if let Some(x) = capture.find('!') {
  - (&capture[..x], &capture[x + 1..])
  + if let Some((n, conv)) = capture.split1_opt('!') {
  + (n, conv)
  } else {
  (capture, "s")
  }
  