fix: fix unity 2019 compatibility

Author: Renardjojo

Runtime/Interface/ILSystemActionVariable.cs

@@ -7,8 +7,8 @@ namespace LSystemPackage
     /// </summary>
     public interface ILSystemActionVariable
     {
-        public abstract char Symbol { get; }
+        char Symbol { get; }
 
-        public abstract void OnSymbolFind();
+        void OnSymbolFind();
     }
 }

Runtime/Interface/ILSystemRule.cs

@@ -7,6 +7,6 @@
     /// </summary>
     public interface ILSystemRule
     {
-        public abstract string SequenceToInsert { get; }
+        string SequenceToInsert { get; }
     }
 }

Runtime/Interface/ILSystemRuleVariable.cs

@@ -7,8 +7,8 @@ namespace LSystemPackage
     /// </summary>
     public interface ILSystemRuleVariable
     {
-        public abstract char Symbol { get; }
+        char Symbol { get; }
 
-        public abstract ILSystemRule Rule { get; }
+        ILSystemRule Rule { get; }
     }
 }

Samples~/Demo/Script/Runtime/LSystemComponent.cs

@@ -10,8 +10,13 @@ namespace LSystemPackage
     [Serializable]
     public class LSystemActionVariable : ILSystemActionVariable
     {
+        [System.Serializable]
+        public class CharEvent : UnityEvent<char>
+        {
+        }
+        
         public string symbol;
-        public UnityEvent<char> action;
+        public CharEvent action;
 
         public char Symbol => symbol.First();