add enumutils.items for sparse enums, typetraits.SomeSparseEnum (#17080)

* add enumutils.items for enum with holes
* changelog
* ref in lib.rst
* use `type SomeSparseEnum* = (not Ordinal) and enum` instead of concept
* address comment: rename back to enum with holes

lib/pure/typetraits.nim

@@ -15,6 +15,17 @@
 import std/private/since
 export system.`$` # for backward compatibility
 
+type SomeEnumWithHoles* = (not Ordinal) and enum ## Enum with holes.
+
+runnableExamples:
+  type A = enum a0 = 2, a1 = 4, a2
+  type B = enum b0 = 2, b1, b2
+  assert A is SomeEnumWithHoles
+  assert B isnot SomeEnumWithHoles
+  assert int isnot SomeEnumWithHoles
+  type C[T] = enum h0 = 2, h1 = 4
+  assert C[float] is SomeEnumWithHoles
+
 proc name*(t: typedesc): string {.magic: "TypeTrait".} =
   ## Returns the name of the given type.
   ##