String escaping in mach.text.escape should be able to write surrogate pairs

[pineapplemachine]

The Escaper type should be able to handle writing code points as \uXXXX\uXXXX surrogate pairs when \xXX and \UXXXXXXXX are not available; it currently will just fail to write the string.

But this is low priority because currently there is no actual use case where this is relevant.