pgrep: Add command `pgrep` #321

Job	Run time
cargo fmt --all -- --check	8s
cargo clippy -- -D warnings (ubuntu-latest)	14s
cargo check (ubuntu-latest)	11s
cargo clippy -- -D warnings (macOS-latest)	22s
cargo check (macOS-latest)	16s
Code Coverage (ubuntu-latest, unix)	2m 4s
cargo clippy -- -D warnings (windows-latest)	1m 37s
Code Coverage (macos-latest, macos)	1m 49s
cargo check (windows-latest)	2m 14s
Code Coverage (windows-latest, windows)	4m 57s
cargo test (ubuntu-latest)	27s
cargo test (macOS-latest)	43s
cargo test (windows-latest)	2m 14s
	17m 16s

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