[ffigen] Should we include -Wno-nullability-completeness compiler option by default?

[dcharkes]

I can't run flutter create --template=plugin_ffi my_package and then dart run ffigen --config ffigen.yaml locally on MacOS without adding:

compiler-opts:
  - '-Wno-nullability-completeness'

We can add this to the Flutter template, but it looks like we're littering this line across the all your ffigen.yamls to make it work in the repository.
So maybe we should just add it by default if it can't do any harm.

[dcharkes]

wdyt @mannprerak2 @liamappelbe ?

[mannprerak2]

Probably yeah, it would reduce a ton of warning logs.

I don't think this flag does anything other than skip warnings. So we probably don't need to increment the major version as well.