Add edition to .rustfmt.toml

When invoked standalone (i.e. not through `cargo fmt`), `rustfmt` does not respect the edition (or any other configuration) in `Cargo.toml`, instead relying solely on `rustfmt.toml` or `.rustfmt.toml`. My editor automatically invokes `rustfmt` on individual files on save. Unless `rustfmt` sees a specified edition, it assumes 2018, and rejects any code that uses `async`. This is a huge pain.

Specify `edition = "2021"` in `.rustfmt.toml` so that standalone `rustfmt` works.

Signed-off-by: Phoebe Goldman <phoebe@clockworklabs.io>

.rustfmt.toml

@@ -1 +1,2 @@
 max_width = 120
+edition = "2021"