feat(macro): add group macro #1099

Job	Run time
Build docs	1m 27s
Test (MSRV)	1m 21s
Clippy	50s
Test (stable)	2m 12s
Rustfmt	12s
Test (beta)	2m 17s
Test (nightly)	1m 53s
Test (macOS)	1m 58s
Test (Windows)	4m 16s
Test (no features)	1m 14s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 25s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 21s
	20m 26s

Provide feedback