Parse error on $ in macro invocations

[ninevra]

Example:

use syn::Token;
use quote::ToTokens;

fn main() {
    let dollar_sign: Token![$] = Default::default();
    println!("{}", dollar_sign.into_token_stream());
}

This is valid, runnable rust code, but does not parse.

This occurs because the macro_invocation grammar production ultimately delegates to _non_special_token, which excludes $. This is the desired bevarior when parsing macro_rules! declarations, where $ is assigned special meaning, but not when parsing other macro invocations, where $ is just a normal token.

This currently breaks Atom rust syntax highlighting for the scope containing the macro invocation and the rest of the document thereafter.