Add security.txt detection with dedicated subcommand

[HuckOps]

feat: Add security.txt detection

1. Add security subcommand to independently output security.txt analysis results
2. Integrate security.txt detection into all command and interactive TUI mode
3. Update long description of root command with security.txt feature notes
4. Implement lookup, field parsing and rendering logic for security.txt

[HuckOps]

[retlehs]

Thanks for the PR, but I don't think security.txt deserves its own top level tab - it's only adopted by a tiny amount of sites