feat: support optional arguments for APPEND command #455

Job	Run time
Clippy check	22s
Checking fmt and docs	32s
Build and test (ubuntu-latest, nightly)	1m 1s
Build and test (ubuntu-latest, beta)	1m 5s
Build and test (ubuntu-latest, stable)	58s
Build and test (windows-latest, nightly)	2m 32s
Build and test (windows-latest, beta)	2m 27s
Build and test (windows-latest, stable)	3m 25s
Build and test (macOS-latest, nightly)	1m 0s
Build and test (macOS-latest, beta)	1m 4s
Build and test (macOS-latest, stable)	1m 9s
	15m 35s

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