Rust DateTime Format With Abbreviated Months In chrono

```cargo
[dependencies]
chrono = "0.4.39"
```

use chrono::prelude::*;

fn main() {
  let date_string = "2003-01-08 04:05:06";
  let dt = NaiveDateTime::parse_from_str(date_string, "%Y-%m-%d %H:%M:%S").unwrap();
  println!("{}", abbreviated_month_string(dt));

  let date_string = "2003-05-08 04:05:06";
  let dt = NaiveDateTime::parse_from_str(date_string, "%Y-%m-%d %H:%M:%S").unwrap();
  println!("{}", abbreviated_month_string(dt));
}

fn abbreviated_month_string(dt: NaiveDateTime) -> String {
  match dt.format("%b").to_string().as_ref() {
    "May" => dt.format("%b %Y").to_string(),
    _ => dt.format("%b. %Y").to_string()
  }
}