raise ArgumentError on unknown directive for Array pack / String unpack #5021

Job	Run time
build native	13m 12s
ruby/spec on CRuby 3.0	3m 32s
build jvm	6m 22s
lint	13m 52s
ruby/spec on CRuby 3.1	3m 28s
ruby/spec on CRuby 3.2	2m 58s
test integration	9m 35s
specs :truffle	14m 47s
fast specs	1m 41s
specs :language :core	12m 32s
specs :library :cext :security :command_line :tracepoint	9m 15s
run MRI tests on native	18m 43s
test native	3m 13s
	1h 53m 10s

Provide feedback