Implement isExported for symbols in macros (#11963)

* Implement isExported for macros

* Reimplement isExported using VM callback mechanism

* VM does not support exceptions, use stacktrace() instead.

lib/core/macros.nim

@@ -1611,3 +1611,6 @@ when defined(nimMacrosSizealignof):
     ## from a field of a type. Therefore it only requires one argument
     ## instead of two. Returns a negative value if the Nim compiler
     ## does not know the offset.
+
+proc isExported*(n: NimNode): bool {.noSideEffect.} =
+  ## Returns whether the symbol is exported or not.