add: basic grammar parser including expression and program.

CanonSharp.Pascal/SyntaxTree/AssignNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class AssignNode(VariableNode variable, SyntaxNodeBase expression) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.Assign;
+
+    public VariableNode Variable => variable;
+
+    public SyntaxNodeBase Expression => expression;
+}

CanonSharp.Pascal/SyntaxTree/BinaryOperatorNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public enum BinaryOperatorType
+{
+    Add,
+    Subtract,
+    Multiply,
+
+    // Pascal特有的整数除法关键词div
+    IntegerDivide,
+    Divide,
+    Mod,
+    And,
+    Or,
+    Equal,
+    NotEqual,
+    Greater,
+    GreaterEqual,
+    Less,
+    LessEqual
+}
+
+public sealed class BinaryOperatorNode(BinaryOperatorType operatorType, SyntaxNodeBase left, SyntaxNodeBase right)
+    : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.BinaryOperator;
+
+    public BinaryOperatorType OperatorType => operatorType;
+
+    public SyntaxNodeBase Left => left;
+
+    public SyntaxNodeBase Right => right;
+}

CanonSharp.Pascal/SyntaxTree/BlockNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class BlockNode(IEnumerable<SyntaxNodeBase> statements) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.Block;
+
+    public IList<SyntaxNodeBase> Statements => statements.ToList();
+}

CanonSharp.Pascal/SyntaxTree/BooleanValueNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class BooleanValueNode(bool value) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.BooleanValue;
+
+    public bool Value => value;
+}

CanonSharp.Pascal/SyntaxTree/FloatValueNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class FloatValueNode(double value) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.FloatValue;
+
+    public double Value => value;
+}

CanonSharp.Pascal/SyntaxTree/IntegerValueNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class IntegerValueNode(int value) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.IntegerValue;
+
+    public int Value => value;
+}

CanonSharp.Pascal/SyntaxTree/Program.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class Program(ProgramHead head, ProgramBody body) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.Program;
+
+    public ProgramHead Head => head;
+
+    public ProgramBody Body => body;
+}

CanonSharp.Pascal/SyntaxTree/ProgramBody.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class ProgramBody(BlockNode block) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.ProgramBody;
+
+    public BlockNode MainBlock => block;
+}

CanonSharp.Pascal/SyntaxTree/ProgramHead.cs

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+using CanonSharp.Pascal.Scanner;
+
+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class ProgramHead(LexicalToken programName) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.ProgramHead;
+
+    public LexicalToken ProgramName => programName;
+}

CanonSharp.Pascal/SyntaxTree/SyntaxNodeBase.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public enum SyntaxNodeType
+{
+    BinaryOperator,
+    UnaryOperator,
+    IntegerValue,
+    FloatValue,
+    BooleanValue,
+    Variable,
+    Assign,
+    Block,
+    ProgramBody,
+    ProgramHead,
+    Program
+}
+
+public abstract class SyntaxNodeBase
+{
+    public abstract SyntaxNodeType NodeType { get; }
+
+    public T Convert<T>() where T : SyntaxNodeBase
+    {
+        if (this is not T result)
+        {
+            throw new InvalidCastException($"Can't convert {NodeType} to target node.");
+        }
+
+        return result;
+    }
+}

CanonSharp.Pascal/SyntaxTree/UnaryOperatorNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public enum UnaryOperatorType
+{
+    Plus,
+    Minus,
+    Not
+}
+
+public sealed class UnaryOperatorNode(UnaryOperatorType operatorType, SyntaxNodeBase node) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.UnaryOperator;
+
+    public UnaryOperatorType OperatorType => operatorType;
+
+    public SyntaxNodeBase Node => node;
+}

CanonSharp.Pascal/SyntaxTree/VariableNode.cs

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+namespace CanonSharp.Pascal.SyntaxTree;
+
+public sealed class VariableNode(string name) : SyntaxNodeBase
+{
+    public override SyntaxNodeType NodeType => SyntaxNodeType.Variable;
+
+    public string IdentifierName => name;
+}