add rustfmt toml (#3491)

* feat: rustfmt.toml

Copied from substrate.

* avoid normalize

It has some odd side effects converting // to /* */ instead of the other way round. See https://github.com/rust-lang/rustfmt/issues/4909 .

polkadot/rustfmt.toml

+# Basic
+hard_tabs = true
+max_width = 100
+use_small_heuristics = "Max"
+# Imports
+imports_granularity = "Crate"
+reorder_imports = true
+# Consistency
+newline_style = "Unix"
+# Misc
+chain_width = 80
+spaces_around_ranges = false
+binop_separator = "Back"
+reorder_impl_items = false
+match_arm_leading_pipes = "Preserve"
+match_arm_blocks = false
+match_block_trailing_comma = true
+trailing_comma = "Vertical"
+trailing_semicolon = false
+use_field_init_shorthand = true