Implement the Default trait with more specific code than the derived Default, for DuplicateFlags.

[rsethc]

This was split off from: #1627

This would be a breaking change since any code that currently uses Default::default() and gets 0, for the types which are receiving a more specific implementation here, will get something different.

It will be a silently breaking change too (code that is broken by this change will likely still compile). It may be preferable to remove the derived Default, instead of implementing it differently, so that code broken by this change catches API users' attention much more noticeably.

Needs:

• Consideration as to whether Default should even remain implemented (with the specialized implementation, as current) or whether the derive should simply be entirely removed instead.
• To have tests added or updated?
• To be squashed.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1630