deprecate do: meaning do (): + misc cleanup (#20927)

* test disable do: block lambda lifting

* fix last test [skip ci]

* deprecate `do:` meaning `do ():` + misc cleanup

closes https://github.com/nim-lang/RFCs/issues/486

* oops

* fix

* no idea what could be causing nimsuggest failure other than this

* ensure ci works

tests/misc/tlambdadonotation.nim

@@ -67,7 +67,7 @@ proc main2() =
   proc foo() =
     subscriber.consume()
 
-  emitter.on_event() do:
+  emitter.on_event() do ():
     subscriber.consume()
 
   # this works