JSON: make unescape_string visible

When using the low-level JSON parser, the caller need a way to
unescape strings.

no-issue-check

core/src/gnatcoll-json.adb

@@ -482,6 +482,26 @@ package body GNATCOLL.JSON is
 
    end Parse_Next;
 
+   ----------------------
+   -- Decode_As_String --
+   ----------------------
+
+   function Decode_As_String
+     (Event : JSON_Parser_Event;
+      Data  : GNATCOLL.Buffer.Reader) return String
+   is
+      Str : String renames
+              Data.Token (Event.First, Event.Last);
+   begin
+      if Event.Kind /= STRING_VALUE then
+         raise Invalid_JSON_Stream with
+           "expected a STRING_VALUE, event is " & Event.Kind'Image;
+      end if;
+
+      return Strings.To_String
+        (Utility.Un_Escape_String (Str, Str'First, Str'Last));
+   end Decode_As_String;
+
    ----------
    -- Read --
    ----------

core/src/gnatcoll-json.ads

@@ -121,6 +121,11 @@ package GNATCOLL.JSON is
       return JSON_Parser_Event;
    --  Fetch the next JSON parser event.
 
+   function Decode_As_String
+     (Event : JSON_Parser_Event;
+      Data  : GNATCOLL.Buffer.Reader) return String;
+   --  Decode a STRING_VALUE as a string.
+
    type JSON_Value_Type is
      (JSON_Null_Type,
       --  Null value: all such JSON values are equivalent