pkgs/profpatsch/nman: ✨ add --file flag to use local nix file

[Profpatsch]

Add support for using ./default.nix instead of when the --local or -l flag is specified. This allows viewing man pages for packages defined in local development environments or custom package sets.

Changes:

• Add use_local field to Main struct
• Parse --local/-l command line option
• Conditionally use ./default.nix in Nix expression generation
• Update help text and man page documentation with new option and examples
• Maintain backward compatibility (default behavior unchanged)

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