add isNamedTuple; make $(1, 2) be (1, 2) instead of (Field0: 1, Field1: 2) which leaked implementation detail (#10070)

* add `isNamedTuple`; make $(1, 2) be (1, 2) instead of leaking implementation detail (Field0: 1, Field1: 2)
  fixes this: #8670 (comment) /cc @alehander42 @Vindaar @mratsim

* Note: isNamedTuple is useful in other places, eg #10010 (comment)

* move isNamedTuple to helpers.nim to avoid exposing new symbol to system.nim

* remove workaround in tests/vm/tissues.nim failing test now that #10218 was makes it work

lib/pure/typetraits.nim

@@ -10,7 +10,10 @@
 ## This module defines compile-time reflection procs for
 ## working with types
 
+include "system/helpers" # for `isNamedTuple`
+
 export system.`$`
+export isNamedTuple
 
 proc name*(t: typedesc): string {.magic: "TypeTrait".}
   ## Alias for system.`$`(t) since Nim v0.20.0.