[CFE][Extension types] No error if the representation type is a bottom type

[sgrekhov]

According to the extension types feature specification

A compile-time error occurs if the representation type of an extension type declaration is a bottom type.

But there is no such error in CFE

extension type ET1(Never _) {} // No error

extension type ET2<T extends Never>(T _) {} // No error

Related analyzer issue #53823

Tested on Dart SDK version: 3.3.0-edge.849459109a364f7f80ca82647ad088a4a6157ebc (be) (Sat Oct 21 02:18:48 2023 +0000) on "macos_x64"