- Add HEIM_JSON_F_ESCAPE_NON_ASCII to indicate that non-ASCII must be
escaped as \uXXXX.
- Add HEIM_JSON_F_NO_ESCAPE_NON_ASCII to force non-escaping of BMP
codepoints.
- If the locale's codeset is not UTF-8 and
HEIM_JSON_F_NO_ESCAPE_NON_ASCII is not set, then set
HEIM_JSON_F_ESCAPE_NON_ASCII.