Move `NODES` constant into its proper namespace

src/ameba/ast/visitors/node_visitor.cr

@@ -1,6 +1,13 @@
 require "./base_visitor"
 
 module Ameba::AST
+  # An AST Visitor that traverses the source and allows all nodes
+  # to be inspected by rules.
+  #
+  # ```
+  # visitor = Ameba::AST::NodeVisitor.new(rule, source)
+  # ```
+  class NodeVisitor < BaseVisitor
     # List of nodes to be visited by Ameba's rules.
     NODES = {
       Alias,
@@ -29,13 +36,6 @@ module Ameba::AST
       Until,
     }
 
-  # An AST Visitor that traverses the source and allows all nodes
-  # to be inspected by rules.
-  #
-  # ```
-  # visitor = Ameba::AST::NodeVisitor.new(rule, source)
-  # ```
-  class NodeVisitor < BaseVisitor
     @skip : Array(Crystal::ASTNode.class)?
 
     def initialize(@rule, @source, skip = nil)