Set rustfmt edition and style_edition for consistency (#2525)

Before, running `rustfmt` on an individual file (eg. via an editor
plugin) would sometimes produce different formatting than running `cargo
fmt` on the whole workspace. That seems to be because `cargo fmt` infers
the `edition` and `style_edition` from Cargo.toml, but `rustfmt` needs
them to be set explicitly in .rustfmt.toml.

Here's a specific formatting difference in case anyone's curious or
wants to test it.

`rustfmt` (before this PR):
```
use foo::{a::C, B};
```
`cargo fmt` (always), and `rustfmt` (after this PR):
```
use foo::{B, a::C};
```

So to be clear, this PR does not change the output of `cargo fmt`. The
only downside I know of is that if we bump the edition of the crates,
we'll need to remember to bump it in .rustfmt.toml too.

Author: evan-oxide

.rustfmt.toml

@@ -1 +1,3 @@
 max_width = 80
+edition = "2024"
+style_edition = "2024"