Handle SEND_MESSAGES_IN_THREADS in required_permissions calculations #1204

Job	Run time
Rustfmt	6s
Build docs	1m 27s
Clippy	52s
Test (MSRV)	1m 23s
Test (stable)	2m 3s
Test (beta)	2m 0s
Test (nightly)	1m 42s
Test (macOS)	2m 19s
Test (Windows)	5m 18s
Test (no features)	1m 10s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 21s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 23s
	21m 4s

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