Remove `Command::type_setter`, replace with `Command::type_` #1159

Job	Run time
Clippy	51s
Build docs	1m 22s
Rustfmt	11s
Test (MSRV)	1m 18s
Test (stable)	1m 57s
Test (beta)	2m 2s
Test (nightly)	1m 39s
Test (macOS)	1m 29s
Test (Windows)	4m 14s
Test (no features)	1m 12s
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 20s
	18m 56s

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