raise ArgumentError on unknown directive for Array pack / String unpack #5019

Job	Run time
build jvm	6m 24s
build native	13m 18s
lint	14m 4s
ruby/spec on CRuby 3.0	3m 27s
ruby/spec on CRuby 3.1	3m 29s
ruby/spec on CRuby 3.2	2m 57s
test integration	9m 41s
fast specs	1m 36s
specs :truffle	14m 40s
specs :language :core	12m 29s
specs :library :cext :security :command_line :tracepoint	9m 36s
test native	3m 16s
run MRI tests on native	19m 28s
	1h 54m 25s

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