Opt-out automatic override keywords introduced by mill-moduledefs

[WojciechMazur]

I personally dislike the fact that mill hides important user facing information by auto-introducing override modifiers using mill-moduledefs compiler plugin. For me information that given value overrides some other defined value is important, and makes the maintenance of project easier.
I wonder, would it be possible to allow for configuration of the plugin/mill to disable this behaviour, so that it would require explicit override modifiers?
I bealive I'm not the only one who'd welcome this change, eg. see VirtusLab/scala-cli which as part of preparation for mill 1.0 upgrade introduced explicit override modifers to protect against source compatibility issues. https://github.com/VirtusLab/scala-cli/pull/3833/files#diff-b0097aeb77849828d4fcfe23679bb71cc487607b4e8a2a7a901e88e74e731e77 Enforcing these is harder since we now cannot get hints from compiler about which methods are actually overrding and which are not.

[lihaoyi]

Sounds reasonable to me, feel free to send a PR to make it opt out!

[lefou]

Being a Mill-specific setting, once landed, we should support it via a dedicated setting/task in MillBuildRootModule. That way, it should also configurable via YAML.