Add `IO::Error#target` #9721

Job	Run time
x86_64-gnu-test (1.7.3)	49m 34s
x86_64-gnu-test (1.8.2)	49m 20s
x86_64-musl-test	1h 2m 29s
x86_64-gnu-test (1.9.2)	38m 32s
x86_64-gnu-test (1.10.1)	47m 6s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	38m 49s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	54m 8s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	40m 12s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	37m 40s
check_format	2m 9s
deploy_api_docs	0s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	41m 46s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	38m 40s
x86_64-gnu-test-preview_mt	8m 32s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	39m 2s
	9h 7m 59s

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