Retain type and field comments in output

See https://github.com/HaxeFoundation/haxe-evolution/pull/65

This requires some busywork in the generators, but other than that shouldn't be controversial.

We need a `-D` flag to allow configuring this behavior.

Start by reviewing haxe-evolution/pull/65, then locate the generators responsible for outputting types and fields. Determine the existing `-D` flag conventions and verify that enabled output retains both type and field comments without changing default behavior.

Written by the indexing model from the issue text.