deps: backport char16ptr.h to 57

• char16ptr.h is appended to the ICU 57 unistr.h
• this way, no v8 code change is needed.

Fixes: https://github.com/nodejs/node/issues/19656

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines