`syn = "2"` support

[MingweiSamuel]

Syn has a new version 2. However the syn-error (and syn) features bring in syn v1.

proc-macro-error/Cargo.toml

Lines 27 to 30 in 75f7d52

	[dependencies.syn]
	version = "1"
	optional = true
	default-features = false

[jayvdb]

syn2 support is available at https://crates.io/crates/proc-macro-error2