Invalid underscore suffix is accepted

The literal suffix cannot be a bare underscore, but proc-macro2 is accepting the following:

```rust
use std::str::FromStr;

fn main() {
for input in [
"'x'_",
"\"s\"_",
"r#\"s\"#_",
] {
println!("{:?}", proc_macro2::TokenStream::from_str(input));
}
}
```

This was an FCW (https://github.com/rust-lang/rust/issues/42326) but was made a hard error in 1.67 (https://github.com/rust-lang/rust/pull/103914).

Using `proc-macro2 1.0.106`.