Edition 2021 reserved prefixed tokens are accepted

The 2021 edition reserved several forms of prefixed literals that proc-macro2 is currently accepting. I'm not sure what the edition policy of proc-macro2 is, but I would somewhat expect these to be rejected.

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

fn main() {
for input in [
"prefix#identifier",
"prefix\"string\"",
"prefix'c'",
"prefix#123",
"'prefix#",
] {
println!("{:?}", proc_macro2::TokenStream::from_str(input));
}
}
```

displays the following:

```
Ok(TokenStream [Ident { sym: prefix }, Punct { char: '#', spacing: Alone }, Ident { sym: identifier }])
Ok(TokenStream [Ident { sym: prefix }, Literal { lit: "string" }])
Ok(TokenStream [Ident { sym: prefix }, Literal { lit: 'c' }])
Ok(TokenStream [Ident { sym: prefix }, Punct { char: '#', spacing: Alone }, Literal { lit: 123 }])
Err(LexError { span: Span })
```

The edition chapter on the reservation: https://doc.rust-lang.org/nightly/edition-guide/rust-2021/reserved-syntax.html. This was from [RFC 3101](https://rust-lang.github.io/rfcs/3101-reserved_prefixes.html). The exact grammar from the reference is:

```
RESERVED_TOKEN_DOUBLE_QUOTE ->
IDENTIFIER_OR_KEYWORD _except `b` or `c` or `r` or `br` or `cr`_ `"`

RESERVED_TOKEN_SINGLE_QUOTE ->
IDENTIFIER_OR_KEYWORD _except `b`_ `'`

RESERVED_TOKEN_POUND ->
IDENTIFIER_OR_KEYWORD _except `r` or `br` or `cr`_ `#`

RESERVED_TOKEN_LIFETIME ->
`'` IDENTIFIER_OR_KEYWORD _except `r`_ `#`
```

(Note, proc-macro2 is handling the lifetime tokens correctly.)

Using `proc-macro2 1.0.106`