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

tests/stdlib/tenumutils.nim

@@ -0,0 +1,16 @@
+discard """
+  targets: "c js"
+"""
+
+import std/enumutils
+from std/sequtils import toSeq
+
+template main =
+  block: # items
+    type A = enum a0 = 2, a1 = 4, a2
+    type B[T] = enum b0 = 2, b1 = 4
+    doAssert A.toSeq == [a0, a1, a2]
+    doAssert B[float].toSeq == [B[float].b0, B[float].b1]
+
+static: main()
+main()