All helpers eventually go through `mkNullOrOption`, so we can move where
`pluginDefault` is handled there.
Added a private helper `processNixvimArgs` that can be used by any future
helper that needs to call `lib.mkOption` directly.
It might make sense to offer a `helpers.mkOption` which simply wraps
`lib.mkOption` but with support for custom args like `pluginDefault`?
a8943f2502