feat(macro): add group macro #1223

Job	Run time
Rustfmt	5s
Test (MSRV)	1m 14s
Build docs	1m 11s
Test (stable)	1m 14s
Clippy	43s
Test (beta)	1m 13s
Test (nightly)	1m 9s
Test (macOS)	1m 25s
Test (Windows)	2m 32s
Test (no features)	1m 3s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 14s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 14s
	14m 17s

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