Return a result on `Ident::new` instead of panicking

I am working on generating code for the wayland protocol in [waybackend](https://codeberg.org/LGFae/waybackend). When creating identifiers, some of them are invalid identifiers in Rust (an enum variant that is just a number). For those, I have been using the following fallback:

```Rust
// prepend an '_' if `name` is an invalid identifier
syn::parse_str(name).unwrap_or_else(|_| syn::parse_str(&format!("_{name}")).unwrap())
```

This is the only use of `syn` in `waybackend-scanner`. If `proc_macro2` returned a Result instead of panicking outright, I could have one less dependency in my project.

I believe this could also be done for some other types as well.

Of course, this would be a breaking change, since old code would break, though you could restore the old behavior by just calling `unwrap`.