Lock semchecked ast for macros (#11883) [bugfix]

* reject to modify type checked AST
* add flag to back out
* Introduce legacy feature set.

tests/macros/tlocktypednode3.nim

@@ -0,0 +1,15 @@
+discard """
+errormsg: "typechecked nodes may not be modified"
+"""
+
+import macros
+
+macro doSomething(arg: typed): untyped =
+  echo arg.treeREpr
+  result = arg
+  result.add(
+    newCall(bindSym"echo", newLit(3)),
+    newCall(bindSym"echo", newLit(1))
+  )
+
+doSomething((echo(1); echo(2)))