Add `EventLoop#wait_[readable|writable]` methods #13331

Job	Run time
check_format	1m 38s
x86_64-gnu-test-preview_mt	6m 8s
x86_64-musl-test	36m 17s
x86_64-gnu-test (1.7.3)	32m 54s
x86_64-gnu-test (1.8.2)	34m 53s
x86_64-gnu-test (1.9.2)	32m 42s
x86_64-gnu-test (1.10.1)	33m 57s
x86_64-gnu-test (1.11.2)	35m 3s
x86_64-gnu-test (1.12.2)	34m 5s
x86_64-gnu-test (1.13.3)	32m 50s
x86_64-gnu-test (1.14.1)	37m 18s
x86_64-gnu-test (1.15.0)	34m 10s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 47s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 41s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	35m 19s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	36m 9s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	33m 12s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	37m 19s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 45s
	9h 56m 7s

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