Experimentally expose RubyVM::AST::Node#node_id

Now ISeq#to_a includes the node_id list for each bytecode instruction.
I want a way to retrieve the AST::Node instance corresponding to an
instruction for a research purpose including TypeProf-based LSP server.

ast.rb

@@ -140,6 +140,17 @@ module RubyVM::AbstractSyntaxTree
       Primitive.ast_node_inspect
     end
 
+    #  call-seq:
+    #     node.node_id -> integer
+    #
+    #  Returns an internal node_id number.
+    #  Note that this is an API for ruby internal use, debugging,
+    #  and research. Do not use this for any other purpose.
+    #  The compatibility is not guaranteed.
+    def node_id
+      Primitive.ast_node_node_id
+    end
+
     #  call-seq:
     #     node.script_lines -> array
     #