buffer: reduce overhead of StringBytes::Encode for UCS2

Currently calling StringBytes::Encode on a UCS2 buffer
results in two copies of the buffer and the usage of
std::vector::assign makes the memory usage unpredictable,
and therefore hard to test against.

This patch makes the memory usage more predictable by
allocating the memory using node::UncheckedMalloc and
handles the memory allocation failure properly. Only
one copy of the buffer will be created and it will
be freed upon GC of the string.

PR-URL: https://github.com/nodejs/node/pull/19798
Refs: https://github.com/nodejs/node/pull/19739


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>