better nativestacktrace support; refs #15284; backport [1.2] (#15384)

* nimStackTraceOverride: enable stack traces in exceptions

This is a two-step stack trace collection scheme, because re-raised
exceptions will collect multiple stack traces but use them rarely, when
printing info about an uncaught exception, so it makes sense to only do
the cheap stack unwinding all the time and the relatively expensive
debugging information collection on-demand.

`asyncfutures` implements its own `$` proc for printing
`seq[StackTraceEntry]`, so we have to add the debugging info there, just
like we do for the private `$` proc in `system/excpt`.

* cleaned up PR #15284

Co-authored-by: Ștefan Talpalaru <stefantalpalaru@yahoo.com>

tests/system/tnim_stacktrace_override.nim

@@ -0,0 +1,18 @@
+discard """
+  cmd: "nim c -d:nimStacktraceOverride $file"
+  output: '''begin
+Traceback (most recent call last, using override)
+Error: unhandled exception: stack trace produced [ValueError]
+'''
+  exitcode: 1
+"""
+
+import asyncfutures
+
+proc main =
+  echo "begin"
+  if true:
+    raise newException(ValueError, "stack trace produced")
+  echo "unreachable"
+
+main()