[ffigen][jnigen] Custom lint for `is` and `as`

It would be nice to warn users they should use `Foo.isA(x)` instead of `x is Foo`, and `Foo.as(x)` instead of `x as Foo`, if `Foo` is an extension type of `ObjCObject`. Same for JNIgen.

https://github.com/dart-lang/native/blob/main/pkgs/ffigen/doc/objc_runtime_types.md