Add a `.rustfmt.toml`

[V0ldek]

Is your feature request related to a problem? Please describe.
rustfmt doesn't behave as I'd like it when it comes to use statements, failing to reorder and appropriately merge them.

Describe the solution you'd like
We should have a custom .rustfmt.toml file that includes these configs:

• imports_granularity
• reorder_impl_items
• reorder_imports
• group_imports
• reorder_modules
• use_field_init_shorthand

Additional context
Configuration docs.

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Tagging @V0ldek for notifications

[V0ldek]

imports_granularity is a very important one, but it's unstable. Blocked on rust-lang/rustfmt#4991