Config `rust-analyzer` to use the new formatting style

OTOH to facilitate use in IDEs so that we can never worry about reorganizing imports, I'm discovering the feasibility of adding a recommended rust-analyzer config that looks like so:

rustfmt = { extraArgs = { "+nightly", "--config-path=.rustfmt.unstable.toml" } },

Interesting, I cannot use that yet because rust-analyzer is calling rustfmt at the source file's basedir so .rustfmt.unstable-toml will be "not found":

• https://github.com/rust-lang/rustfmt/issues/4660

Originally posted by @rami3l in #5065