Fix rustfmt for files that use 2024-edition syntax

"cargo fmt" works fine but "rustfmt" fails to format some files.

	$ rustfmt crates/ide-db/src/search.rs
	error: let chains are only allowed in Rust 2024 or later
	   --> /home/johannes/git/rust-analyzer/crates/ide-db/src/search.rs:298:12
	    |
	298 |         if let &Definition::Module(module) = self
	    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I guess I could work around this by setting my format command to
"cargo fmt -- $filename" instead of "rustfmt $filename".

But it'd be nice if this worked OOTB. Make it so by adding specifying
the edition in rustfmt.toml.  We already have several other places
specifying the edition.

changelog internal

Author: krobelus