Add :required for :flags

[gabriel376]

Solves #4

Notes

• Make (parse-error!) thrown an Exception rather than calling (System/exit), so tests can be executed and checked against it
• Split the main dispatch function into (dispatch*) and (dispatch): the former contains the actual logic and the latter handles exceptions
• Fixes a small duplicated blank space when printing Unknown flag:
• In case of an Exception is caught, make (dispatch) print FATAL messages to stderr
• Add tests

[plexus]

Why did you get rid of the original tests? Were they failing?

[plexus]

Released in v0.8.39

[com.lambdaisland/cli "0.8.39"]                 ;; deps.edn
{com.lambdaisland/cli {:mvn/version "0.8.39"}}  ;; project.clj