implement `pretty_help` #957

Job	Run time
Clippy	50s
Build docs	1m 24s
Test (MSRV)	1m 16s
Rustfmt	5s
Test (stable)	1m 41s
Test (beta)	1m 25s
Test (nightly)	1m 52s
Test (macOS)	1m 59s
Test (Windows)	3m 49s
Test (no features)	1m 15s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 11s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 25s
	18m 12s

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