Add `StringLiteral#to_utf16` #11546

Job	Run time
check_format	1m 49s
deploy_api_docs	0s
x86_64-gnu-test (1.7.3)	33m 32s
x86_64-gnu-test-preview_mt	5m 47s
x86_64-gnu-test (1.8.2)	33m 6s
x86_64-gnu-test (1.9.2)	35m 5s
x86_64-musl-test	38m 4s
x86_64-gnu-test (1.10.1)	33m 6s
x86_64-gnu-test (1.11.2)	34m 4s
x86_64-gnu-test (1.12.2)	36m 58s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 20s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 28s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	32m 55s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	35m 8s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	33m 58s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	35m 17s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	33m 12s
	8h 7m 49s

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