Add .rustfmt.toml (#369)

Unlike `cargo fmt`, `rustfmt` is agnostic to `Cargo.toml` and does not
pick up the edition from within that file. Instead, `rustfmt` decides
what edition to use based on what is specified inside of `.rustfmt.toml`
or if the edition is explicitly passed into the CLI command.

Having this file helps contributors who have configured auto formatting,
on file save or the like, using `rustfmt` rather than `cargo fmt`.

Author: Nydauron

.rustfmt.toml

@@ -0,0 +1 @@
+edition = "2021"