refactor: remove structopt in favour of clap (#591)

* refactor: remove structopt in favour of clap

as per
[`structopt` README](https://github.com/TeXitoi/structopt#maintenance),
it is now in maintenance mode.

migrating to `clap` v3 as per their
[migration docs.](https://github.com/clap-rs/clap/blob/master/CHANGELOG.md#300---2021-12-31).

* refactor: upgrade clap to v4

implement `clap` breaking changes as per docs.

* test: add parameterized tests for nmap-command parsing

Author: PsypherPunk