Add ignore_thread_creation to PrefixFrameworkOptions #882

Job	Run time
Test (MSRV)	6m 24s
Build docs	1m 47s
Clippy	1m 6s
Rustfmt	9s
Test (stable)	3m 33s
Test (beta)	3m 48s
Test (nightly)	3m 34s
Test (macOS)	2m 37s
Test (Windows)	7m 31s
Test (no features)	1m 53s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 49s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	2m 26s
	36m 37s

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