Handle SEND_MESSAGES_IN_THREADS in required_permissions calculations #1207

Job	Run time
Build docs	1m 33s
Test (MSRV)	1m 22s
Rustfmt	6s
Test (stable)	1m 49s
Clippy	54s
Test (beta)	1m 59s
Test (nightly)	1m 46s
Test (macOS)	1m 44s
Test (Windows)	4m 27s
Test (no features)	1m 11s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 19s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 15s
	19m 25s

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