Added codeRepr and dumpCode to the macros module. (#5778)

tests/macros/tdumpastgen.nim

@@ -0,0 +1,25 @@
+discard """
+msg: '''nnkStmtList.newTree(
+  nnkVarSection.newTree(
+    nnkIdentDefs.newTree(
+      newIdentNode(!"x"),
+      newEmptyNode(),
+      nnkCall.newTree(
+        nnkDotExpr.newTree(
+          newIdentNode(!"foo"),
+          newIdentNode(!"create")
+        ),
+        newLit(56)
+      )
+    )
+  )
+)'''
+"""
+
+# disabled; can't work as the output is done by the compiler
+
+import macros
+
+dumpAstGen:
+  var x = foo.create(56)
+