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.

tests/macros/tisexported.nim

@@ -0,0 +1,10 @@
+import macros
+
+proc t1* = discard
+proc t2 = discard
+
+macro check(p1: typed, p2: typed) =
+  doAssert isExported(p1) == true
+  doAssert isExported(p2) == false
+
+check t1, t2