feat(parser): Generate nullable fields, rename canBeNull to optional

[netux]

This is different from optional fields (field?: type) in that we can use null as a value instead of writing the property altogether.

For example:

interface MyType {
  optionalField?: boolean;
  nullableField: boolean | null;
  optionalAndNullableField?: boolean | null;
}

const obj: MyType = {
  // no optionalField here is fine, TypeScript won't complain

  nullableField: null, // this is fine as well, TypeScript won't complain

  optionalAndNullableField: null // this is fine as well, but we could also not include this field and TypeScript wouldn't complain either
}

Renamed old canBeNull to optional to be able to reuse canBeNull for the new functionality.

I didn't know what to do on action.func.ts, so I just defaulted canBeNull to false.