fixes #5986 js backend failed to compile try ... except new syntax (#6116)

tests/js/ttryexceptnewsyntax.nim

@@ -0,0 +1,13 @@
+discard """
+  output: '''hello'''
+"""
+
+type
+  MyException = ref Exception
+
+#bug #5986
+
+try:
+  raise MyException(msg: "hello")
+except MyException as e:
+  echo e.msg