feat(AST): Add node factory helper and missing identifier node

Signed-off-by: erick-alcachofa <erick@artichoke.dev>

This commit introduces an utility factory function and structural
improvements to the Abstract Syntax Tree (AST).

* Adds a new `ASTNodePtr` C++20 concept to constrain template types to
  be `std::unique_ptr`s pointing to AST nodes.
* Introduces a `MakeNode<T>()` factory function that uses this concept
  to simplify and standardize the creation of new nodes.
* Fixed `NamespacedType` and added the missing `NamespacedIdentifier`
  node.

lib/include/artichoke/Parser/AST/AST.hpp

@@ -23,4 +23,9 @@ namespace arti::lang::ast {
     Vector<TopLevelDeclNode> declarations;
   };
 
+  template <ASTNodePtr Node>
+  auto MakeNode() {
+    return std::make_unique<typename Node::element_type>();
+  }
+
 } // namespace arti::lang::ast

lib/include/artichoke/Parser/AST/Common.hpp

@@ -74,4 +74,10 @@ namespace arti::lang::ast {
   template <typename T>
   using Vector = std::vector<T>;
 
+  template <typename Node>
+  concept ASTNodePtr = requires {
+    typename Node::element_type;
+    requires std::is_same_v<std::unique_ptr<typename Node::element_type>, Node>;
+  };
+
 } // namespace arti::lang::ast

lib/include/artichoke/Parser/AST/Types.hpp

@@ -57,6 +57,13 @@ namespace arti::lang::ast {
   struct nodes::NamespacedType {
     SourceLocation location;
 
+    String typeName;
+    TypeExpressionNode baseType;
+  };
+
+  struct nodes::NamespacedIdentifier {
+    SourceLocation location;
+
     Vector<String> identParts;
   };