Support for the special `$crate` identifier

In my [custom-format crate](https://github.com/x-hgg-x/custom-format/blob/bdc31a66d7940c3c4ed5680271f86fca1c70cde9/src/lib.rs#L44), I pass the special `$crate` identifier from a declarative macro to a procedural macro, which allows me to avoid using a crate such as [proc-macro-crate](https://crates.io/crates/proc-macro-crate). This identifier cannot be created with the `Ident::new` method, so the procedural macro cannot be tested with real inputs using the `proc_macro2` crate.

Is it acceptable to extend this crate to allow constructing the `$crate` identifier via
```rust
proc_macro2::Ident::new("$crate", Span::call_site())
```
instead of panicking ?