Update syn to v2

[edwloef]

Fairly straightforward update, effectively only the parse_nested_meta change. All tests pass locally.

If you'd like I can drop the linting/formatting commit, in case it causes too much noise.

[edwloef]

Note: I didn't see this at first, but this could warrant an msrv bump from 1.60 to 1.61 since syn requires rust 1.61 since version 2.0.68. Alternatively people using rust 1.60 could just use an older compatible version of syn 2 🤷 Never mind, #11 already requires rust 1.61 to compile