Fix unescaping of JSON Unicode escapes, especially for non-UTF8.

Per discussion  on -hackers. We treat Unicode escapes when unescaping
them similarly to the way we treat them in PostgreSQL string literals.
Escapes in the ASCII range are always accepted, no matter what the
database encoding. Escapes for higher code points are only processed in
UTF8 databases, and attempts to process them in other databases will
result in an error. \u0000 is never unescaped, since it would result in
an impermissible null byte.

src/test/regress/sql/json.sql

@@ -299,8 +299,14 @@ select * from json_populate_recordset(row('def',99,null)::jpop,'[{"c":[100,200,3
 
 -- handling of unicode surrogate pairs
 
-select json '{ "a":  "\ud83d\ude04\ud83d\udc36" }' -> 'a' as correct;
+select json '{ "a":  "\ud83d\ude04\ud83d\udc36" }' -> 'a' as correct_in_utf8;
 select json '{ "a":  "\ud83d\ud83d" }' -> 'a'; -- 2 high surrogates in a row
 select json '{ "a":  "\ude04\ud83d" }' -> 'a'; -- surrogates in wrong order
 select json '{ "a":  "\ud83dX" }' -> 'a'; -- orphan high surrogate
 select json '{ "a":  "\ude04X" }' -> 'a'; -- orphan low surrogate
+
+--handling of simple unicode escapes
+
+select json '{ "a":  "the Copyright \u00a9 sign" }' ->> 'a' as correct_in_utf8;
+select json '{ "a":  "dollar \u0024 character" }' ->> 'a' as correct_everywhere;
+select json '{ "a":  "null \u0000 escape" }' ->> 'a' as not_unescaped;