Stmt: Introduce STMT_EXTERN

It's currently possible for plugin's to implement their own statement
subclasses and override the Exec() implementation. This has been leveraged
by ZeekJS [1] and zeek-perf-support [2] as well as a private WASM plugin.
All of these used STMT_ANY as the tag of their own statement subclasses.

With STMT_EXTERN, we make the possibility to add external code into the AST
somewhat more supported. It's all in detail space and plugin authors have
no guarantee for stability, but it seems such a powerful extension point
that IMO we should keep it.

I'm conscious there's the broader topic how this interacts with ZAM
optimization like in-lining or rewriting of statements. However, this
already applies to the STMT_ANY usage of the mentioned plugins.

[1] https://github.com/corelight/zeekjs
[2] https://github.com/zeek/zeek-perf-support

src/Stmt.cc

@@ -52,6 +52,7 @@ const char* stmt_name(StmtTag t) {
         "ZAM",
         "null",
         "assert",
+        "extern",
     };
 
     return stmt_names[int(t)];