[FEATURE]: Add nixfmt formatter for Nix files #6379

Summary

Add support for nixfmt as a built-in formatter for .nix files.

Context

• nixfmt is the official Nix formatter (RFC 166)
• OpenCode already has LSP support for Nix via nixd, but no formatter
• As of NixOS 25.11, pkgs.nixfmt is the stable RFC-style formatter

Proposed Solution

Add nixfmt to the built-in formatters following the existing pattern (like gofmt, gleam, shfmt):

• Extensions: .nix
• Command: nixfmt $FILE
• Enabled when nixfmt is available in PATH