Handle SEND_MESSAGES_IN_THREADS in required_permissions calculations #1203

Job	Run time
Clippy	49s
Build docs	1m 26s
Rustfmt	14s
Test (MSRV)	1m 15s
Test (stable)	1m 58s
Test (beta)	2m 0s
Test (nightly)	1m 40s
Test (macOS)	1m 53s
Test (Windows)	4m 21s
Test (no features)	1m 6s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 15s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 14s
	19m 11s

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