Add toolchain and format config

rust-toolchain.toml

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+[toolchain]
+channel = "nightly"

treefmt.toml

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+# One CLI to format the code tree - https://github.com/numtide/treefmt
+
+[formatter.nix]
+command = "alejandra"
+includes = [ "*.nix" ]
+
+[formatter.rust]
+command = "rustfmt"
+options = ["--edition", "2021"]
+includes = ["*.rs"]