Set rustfmt edition and style_edition for consistency

[evan-oxide]

This is equivalent to this hubris PR: oxidecomputer/hubris#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 workspace, we'll need to remember to bump it in .rustfmt.toml too.