feat: add experimental shpool-vterm engine #598

Job	Run time
cargo deny --all-features check licenses	59s
cargo test --all-features (ubuntu-22.04)	1m 26s
cargo +nightly fmt -- --check	1m 12s
cargo test --all-features (macos-latest)	2m 10s
cargo +nightly cranky --all-targets -- -D warnings	1m 0s
	6m 47s

Provide feedback