Fix and improve list parser of cabal init cli (#8663)

* Fix cli list parse errors in `cabal init`

Occurrences of `Flag [a]` behave in a slightly unexpected way. The monoid
instance of `Flag` is right associative and discard the value on the
left.
Thus, make sure we merge the contents of the flags, instead of using the
monoid instance of `Flag` itself.

* Document fixes and improvements

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>