Add support for UTF-32BE

[lemire]

Currently, we refer to UTF-32LE as UTF-32. We need to add support UTF-32BE which is relatively easy.

Related: https://github.com/simdutf/simdutf/issues/3

[lemire]

It is probably more urgent to address https://github.com/simdutf/simdutf/issues/3

There is probably little need for UTF-32BE since UTF-32 is mostly used internally, and processors are invariably LE these days.

cc @NicolasJiaxin