rustfmt.toml: group_imports = StdExternalCrate

Summary:
To be landed when reformat changes land.

[Context](https://fb.workplace.com/groups/rust.language/posts/9143028552412345).

Reviewed By: zertosh

Differential Revision: D38485268

fbshipit-source-id: 2a193d675b60f67860fc437b0e2ee4982af8706f

rustfmt.toml

@@ -1,6 +1,7 @@
 # Get help on options with `rustfmt --help=config`
 # Please keep these in alphabetical order.
 edition = "2021"
+group_imports = "StdExternalCrate"
 imports_granularity = "Item"
 merge_derives = false
 use_field_init_shorthand = true