Make async stacktraces less verbose (#21091)

* Name iterators something human readable

Remove intermediate async procs from stacktraces

Clean async traceback message from reraises message

* Remove unused import/variable

* Fix failing tests

Don't add {.stackTrace: off.} to anonymous procs (They already don't appear in stacktrace)

* Fix failing tests in pragma category

Now check that the nim is a routine type first so we don't run into any assertion defects

* Hide stack trace pragma in docs and update doc tests

User doesn't need to know if something won't appear so this more becomes verbose noise

If this is a bad idea we can always add a `when defined(nimdoc)` switch so we don't add {.stackTrace: off.} to the Future[T] returning proc for docs

tests/async/tasync_gcunsafe.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "'anotherGCSafeAsyncProcIter' is not GC-safe as it calls 'asyncGCUnsafeProc'"
+  errormsg: "'anotherGCSafeAsyncProc (Async)' is not GC-safe as it calls 'asyncGCUnsafeProc'"
   cmd: "nim c --threads:on $file"
   file: "asyncmacro.nim"
 """