Add nixica implementation

Summary

• Implement lib.mkFlake helper to simplify flake boilerplate
• Add nixica.formatting module with Nix formatters (nixfmt, deadnix, statix, typos)
• Bundle flake-parts, import-tree, and treefmt-nix as dependencies
• Add comprehensive documentation and examples
• Include test flake demonstrating usage

Benefits

• Reduces flake boilerplate from 4+ inputs to 2 inputs
• Simplifies outputs line to single function call
• Provides optional formatting with one-line enable
• Deduplicates dependencies across multiple flakes

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