Support escaping only "special" non-ASCII characters

[jwodder]

There should be an option to serialize non-ASCII characters in strings as-is, except for control characters, unassigned characters, and similar, which should be encoded as \uXXXX escape sequences.

• Possible set of characters to escape: all those with general category Cc, Cs, Co, or Cn? (i.e., all C other than Cf)
  • Cf. https://en.wikipedia.org/w/index.php?title=Graphic_character&oldid=1254105272#Unicode