Set up ast and nodes for the feature set

include/dragon/ast.h

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+#pragma once 
+
+#include <string>
+#include <memory>
+#include <vector>
+#include <variant>
+#include "token.h"
+
+class Node {
+public:
+    virtual ~Node() = default;
+    virtual std::string to_string() const = 0;
+};
+
+class ExpressionNode : public Node {
+public:
+    Token op;
+    std::unique_ptr<Node> left;
+    std::unique_ptr<Node> right;
+
+    ExpressionNode(Token token, std::unique_ptr<Node> left, std::unique_ptr<Node> right) 
+        : op(std::move(token)), left(std::move(left)), right(std::move(right)) {}
+
+    ~ExpressionNode() override = default;
+
+    std::string to_string() const override {
+        return "ExpressionNode(op: " + op.to_string() + ")";
+    }
+
+    virtual std::variant<int, std::string, bool> eval() const = 0;
+    virtual size_t size() const = 0;
+};
+
+class StatementNode : public Node {
+public:
+    virtual ~StatementNode() override = default;
+    virtual std::string to_string() const override = 0; 
+};
+
+

include/dragon/ast/for_loop_node.h

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+#pragma once 
+
+#include "../ast.h"
+
+class ForLoopNode : public StatementNode {};

include/dragon/ast/function_call_node.h

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+#pragma once 
+
+#include "../ast.h"
+
+class FunctionCallNode : public ExpressionNode {};

include/dragon/ast/identifier_node.h

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+#pragma once 
+
+#include "../ast.h"
+
+class IdentifierNode : public ExpressionNode {};

include/dragon/ast/if_else_node.h

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+#pragma once 
+
+#include "../ast.h"
+
+class IfElseNode : public StatementNode {};

include/dragon/ast/literal_node.h

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+#pragma once 
+
+#include "../ast.h"
+
+class LiteralNode : public ExpressionNode {};

include/dragon/ast/variable_assignment_node.h

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+#pragma once 
+
+#include "../ast.h"
+
+class VariableAssignmentNode : public StatementNode {};

include/dragon/ast/variable_declaration_node.h

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+#pragma once 
+
+#include <memory>
+#include "../ast.h"
+
+class VariableDeclarationNode : public StatementNode {
+public:
+    Token identifier;
+    Token type;
+    std::unique_ptr<ExpressionNode> value;
+    bool is_const;
+
+    explicit VariableDeclarationNode(Token identifier, Token type, std::unique_ptr<ExpressionNode> value, bool is_const) 
+        : identifier(std::move(identifier)), type(std::move(type)), value(std::move(value)), is_const(is_const) {}
+
+    ~VariableDeclarationNode() override = default;
+
+    std::string to_string() const override {
+        return "VariableDeclarationNode(identifier: " + identifier.to_string()
+            + ", type: " + type.to_string() 
+            + ", is_const: " + (is_const ? "true" : "false") + ")";
+    }
+};

include/dragon/ast/while_loop_node.h

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+#pragma once 
+
+#include "../ast.h"
+
+class WhileLoopNode : public StatementNode {};