Implement rb_enc_interned_str #4961

Job	Run time
build native	13m 11s
lint	14m 2s
build jvm	6m 16s
ruby/spec on CRuby 3.0	3m 30s
ruby/spec on CRuby 3.1	3m 31s
ruby/spec on CRuby 3.2	2m 55s
test integration	9m 1s
specs :truffle	14m 42s
fast specs	1m 58s
specs :language :core	12m 40s
specs :library :cext :security :command_line :tracepoint	9m 26s
test native	3m 5s
run MRI tests on native	15m 49s
	1h 50m 6s

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