Add firewall subcommand

[PheonixScale9094]

firewall: Subcommand to perform firewall actions. Initial functionality: List open ports in the firewall. Color them if there's an existing service behind them. Ought to function across iptables and nft

[judah-sotomayor]

Script Progression Goals

Version 1

• Print IPTables or NFTables rules when you run jj fw or jj firewall
• Then list ports that services are listening on

Version 2

• Write a nice perlfmt to pretty-print firewall rules.
• Color ports green or blue or something if there's a service behind them

Version 3

• Basic rule-making capabilities?
• Color ports differently depending on whether a proper service is listening or just a process

[judah-sotomayor]

Another requirement: Make sure ICMP is allowed by default.