'isolate' builtin; refs https://github.com/nim-lang/RFCs/issues/244 (#15011)

tests/isolate/tisolate.nim

@@ -0,0 +1,36 @@
+discard """
+  errormsg: "expression cannot be isolated: select(a, b)"
+  line: 34
+"""
+
+import std / isolation
+
+import json, streams
+
+proc f(): seq[int] =
+  @[1, 2, 3]
+
+type
+  Node = ref object
+    x: string
+
+proc g(): Node = nil
+
+proc select(a, b: Node): Node =
+  a
+
+proc main =
+  discard isolate f()
+
+
+  discard isolate g()
+
+  discard isolate select(Node(x: "a"), nil)
+  discard isolate select(Node(x: "a"), Node(x: "b"))
+
+  discard isolate parseJson(newFileStream("my.json"), "my.json")
+
+  var a, b: Node
+  discard isolate select(a, b)
+
+main()