Prevent `tauri::ipc::Channel` outside function arguments and results

[oscartbeaumont]

I don't think it's valid so we should detect and error.

The current prototype I had is the following:

// TODO: This only works if no functions use `Channel` which isn't really desired.
// TODO: I wonder if this error should be in Specta???
// TODO: Test this works
// if !is_channel_used
//     && let Some(ndt) = builder.types.into_unsorted_iter().find(|ndt| {
//         ndt.name() == "TAURI_CHANNEL" && ndt.module_path().starts_with("tauri::")
//     })
// {
//     return Err(Error::Framework(
//         format!("Detected invalid usage of `tauri::ipc::Channel` in type '{}' defined in {} which is not valid! It can only be an argument or result of a Tauri command.", ndt.name(), ndt.module_path()).into()
//     ));
// }

but this overlooks the fact that the channel can be included in a struct & the channel import can be used in a function and it would let it through. It needs to be smarter.

Can a Tauri channel be included in an event??? Add that into detection code.