Split off from #19151

I found an issue in v8 which I worked around at srl295@384ab7d - introduced into v8 at v8/v8@99e8963 (and into node later) - there is a call to icu::toUCharPtr() passed as an input to a reinterpret_cast. This seems to unnecessarily tie the code to ICU 59+, and in any event ICU uses char16_t and not UChar as the type for C++ going forward. So toUCharPtr() is not something to call going forward. Background here

If this were fixed, ICU4C 57 ought to be supported by master at this point.