perf(derive): Improve build times

[epage]

The more widely available improvement is the dropping of proc-macro-error. This dependency was less than ideal because it itself was a proc macro, further serializing our builds (requiring all proc macro machiner to be built before proc-macro-error before clap_derive). Now, we don't just have fewer dependencies but hopefully more parallelism.

This unblock the second almost as widely scoped improvement: upgrading to syn v2. proc-macro-error was on syn v1 and it hadn't been touched in a while, so I wasn't confident in a timely upgrade (plus I've been wanting to drop it anyways, see above). Now, it'll be easier to only have one version of syn in the dependency tree (as of 5 days ago, 39.3% of all crates depended on syn v2 already)