refactor: simplify fspy::Command API and add pre_exec hook #51

Job	Run time
Test (namespace-profile-mac-default, aarch64-apple-darwin)	27s
Lint	1m 23s
Test (windows-latest, x86_64-pc-windows-msvc)	2m 5s
Test (ubuntu-latest, x86_64-unknown-linux-gnu)	1m 9s
done	0s
	5m 4s

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