added enum indexed array support to json (#16807)

* added enum indexed array support to json

* Added json test

* Removed when statement for enum indexed arrays

tests/stdlib/tjson_enum_index.nim

@@ -0,0 +1,11 @@
+import json
+type Test = enum
+  one, two, three, four, five
+let a = [
+  one: 300,
+  two: 20,
+  three: 10,
+  four: 0,
+  five: -10
+]
+doAssert (%* a).to(a.typeof) == a