json: add '\v' support, fixes #10541 (#10987)

lib/pure/json.nim

@@ -636,6 +636,7 @@ proc escapeJsonUnquoted*(s: string; result: var string) =
     of '\b': result.add("\\b")
     of '\f': result.add("\\f")
     of '\t': result.add("\\t")
+    of '\v': result.add("\\v")
     of '\r': result.add("\\r")
     of '"': result.add("\\\"")
     of '\0'..'\7': result.add("\\u000" & $ord(c))

lib/pure/parsejson.nim

@@ -217,6 +217,9 @@ proc parseString(my: var JsonParser): TokKind =
       of 't':
         add(my.a, '\t')
         inc(pos, 2)
+      of 'v':
+        add(my.a, '\v')
+        inc(pos, 2)
       of 'u':
         if my.rawStringLiterals:
           add(my.a, 'u')