Add support for deriving `Default` for "shared" structs

[baylesj]

Currently, cxx does not support #[derive(Default)] for shared structs, resulting in the following error message:

error[cxxbridge]: derive(Default) on shared enum is not supported
   ┌─ ../../path/to/file:line
   │
99 │     #[derive(Debug, Clone, Copy, Default)]
   │                                  ^^^^^^^ derive(Default) on shared enum is not supported

It would be nice if the Default derivation case was supported.

[anforowicz]

Thanks for opening an issue. I guess the expectation here is that the same syntax would work as outside of #[cxx::bridge] - e.g. (Rust playground link):

#[derive(Debug, Default)]
pub enum Foo {
    #[default]
    Bar,
    Baz,
}