Add mermaid graph format

src/Stateless/Graph/MermaidGraph.cs

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+using Stateless.Reflection;
+
+namespace Stateless.Graph
+{
+    /// <summary>
+    /// Class to generate a MermaidGraph
+    /// </summary>
+    public static class MermaidGraph
+    {
+        /// <summary>
+        /// Generate a Mermaid graph from the state machine info
+        /// </summary>
+        /// <param name="machineInfo"></param>
+        /// <returns></returns>
+        public static string Format(StateMachineInfo machineInfo)
+        {
+            var graph = new StateGraph(machineInfo);
+
+            return graph.ToGraph(new MermaidGraphStyle());
+        }
+
+    }
+}

src/Stateless/Graph/MermaidGraphStyle.cs

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+using Stateless.Reflection;
+using System;
+using System.Collections.Generic;
+using System.Reflection.Emit;
+using System.Text;
+
+namespace Stateless.Graph
+{
+    /// <summary>
+    /// Class to generate a graph in mermaid format
+    /// </summary>
+    public class MermaidGraphStyle : GraphStyleBase
+    {
+        /// <summary>
+        /// Returns the formatted text for a single superstate and its substates.
+        /// For example, for DOT files this would be a subgraph containing nodes for all the substates.
+        /// </summary>
+        /// <param name="stateInfo">The superstate to generate text for</param>
+        /// <returns>Description of the superstate, and all its substates, in the desired format</returns>
+        public override string FormatOneCluster(SuperState stateInfo)
+        {
+            string stateRepresentationString = "";
+            return stateRepresentationString;
+        }
+
+        /// <summary>
+        /// Generate the text for a single decision node
+        /// </summary>
+        /// <param name="nodeName">Name of the node</param>
+        /// <param name="label">Label for the node</param>
+        /// <returns></returns>
+        public override string FormatOneDecisionNode(string nodeName, string label)
+        {
+            return String.Empty;
+        }
+
+        /// <summary>
+        /// Generate the text for a single state
+        /// </summary>
+        /// <param name="state">The state to generate text for</param>
+        /// <returns></returns>
+        public override string FormatOneState(State state)
+        {
+            return String.Empty;
+        }
+
+        /// <summary>Get the text that starts a new graph</summary>
+        /// <returns></returns>
+        public override string GetPrefix()
+        {
+            return "stateDiagram-v2";
+        }
+
+        /// <summary>
+        /// 
+        /// </summary>
+        /// <param name="initialState"></param>
+        /// <returns></returns>
+        public override string GetInitialTransition(StateInfo initialState)
+        {
+            return $"\r\n[*] --> {initialState}";
+        }
+
+
+
+        /// <summary>
+        /// 
+        /// </summary>
+        /// <param name="sourceNodeName"></param>
+        /// <param name="trigger"></param>
+        /// <param name="actions"></param>
+        /// <param name="destinationNodeName"></param>
+        /// <param name="guards"></param>
+        /// <returns></returns>
+        public override string FormatOneTransition(string sourceNodeName, string trigger, IEnumerable<string> actions, string destinationNodeName, IEnumerable<string> guards)
+        {
+            string label = trigger ?? "";
+
+            return FormatOneLine(sourceNodeName, destinationNodeName, label);
+        }
+
+        internal string FormatOneLine(string fromNodeName, string toNodeName, string label)
+        {
+            return $"\t{fromNodeName} --> {toNodeName} : {label}";
+        }
+    }
+}

test/Stateless.Tests/MermaidGraphFixture.cs

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+using Xunit;
+
+namespace Stateless.Tests
+{
+    public class MermaidGraphFixture
+    {
+        [Fact]
+        public void Format_InitialTransition_ShouldReturns()
+        {
+            var expected = "stateDiagram-v2\r\n[*] --> A";
+
+            var sm = new StateMachine<State, Trigger>(State.A);
+
+            var result = Graph.MermaidGraph.Format(sm.GetInfo());
+
+            Assert.Equal(expected, result);
+
+        }
+
+        [Fact]
+        public void Format_SimpleTransition()
+        {
+            var expected = "stateDiagram-v2\r\n\tA --> B : X\r\n[*] --> A";
+
+            var sm = new StateMachine<State, Trigger>(State.A);
+
+            sm.Configure(State.A)
+                .Permit(Trigger.X, State.B);
+
+            var result = Graph.MermaidGraph.Format(sm.GetInfo());
+
+            Assert.Equal(expected, result);
+
+        }
+
+        [Fact]
+        public void TwoSimpleTransitions()
+        {
+            var expected = """
+        stateDiagram-v2
+            A --> B : X 
+            A --> C : Y
+        """;
+
+            var sm = new StateMachine<State, Trigger>(State.A);
+
+            sm.Configure(State.A)
+                   .Permit(Trigger.X, State.B)
+                   .Permit(Trigger.Y, State.C);
+
+            var result = Graph.MermaidGraph.Format(sm.GetInfo());
+
+            Assert.Equal(expected, result);
+
+        }
+    }
+}