Consider switching to syn 2

[daniel-noland]

We are using this crate over at Hedgehog's dataplane (or will be once https://github.com/githedgehog/dataplane/pull/392 lands).

That said, our CI is bouncing this crate as a dependency for two reasons.

• The first is RUSTSEC-2024-0370 addressed in #52
• The second is that the use of syn v1 is introducing duplicate dependencies (which are soft banned by our CI).

I am happy to help switch this crate over to syn2, but I wanted to get feedback to clarify that this is desired

[lun3x]

Thanks for taking a look, indeed proc-macro-error does seem to be abandoned. I've checked that the suggested replacement and all looks good. I'll approve your PR for that now.

I'm happy to accept PRs to upgrade to syn2, proc-macro-error was a blocker here. I also didn't require any new features of syn2 so I hadn't looked into upgrading yet, but I think it should be fairly straight forward.