feat: Parser Combinator库和词法分析器 (#2)

Reviewed-on: https://git.bupt-hpc.cn/jackfiled/CanonSharp/pulls/2
Co-authored-by: jackfiled <xcrenchangjun@outlook.com>
Co-committed-by: jackfiled <xcrenchangjun@outlook.com>

CanonSharp.Combinator/Unit.cs

@@ -0,0 +1,23 @@
+namespace CanonSharp.Combinator;
+
+/// <summary>
+/// 单元类型Unit
+/// </summary>
+public readonly struct Unit : IComparable<Unit>, IEquatable<Unit>
+{
+    public static Unit Instance => default;
+
+    public bool Equals(Unit other) => true;
+
+    public int CompareTo(Unit other) => 0;
+
+    public override bool Equals(object? obj) => obj is Unit;
+
+    public override int GetHashCode() => 0;
+
+    public override string ToString() => $"<{nameof(Unit)}>";
+
+    public static bool operator ==(Unit _0, Unit _1) => true;
+
+    public static bool operator !=(Unit _0, Unit _1) => false;
+}