fix(codegen): Creates multiple steps for serialization source generator (#628)

Roslyn Source generators are not supposed to access I/O. To get around this we have to use `AdditionalFiles` to give the analyzer access to read/load the schema files.
To write the schema files we have to use a separate program altogether.

- [X] Adds SerializationSchemaGenerator
- [X] Splits out the SerializationGenerator code

Projects/SerializationSchemaGenerator/SyntaxVisitor.cs

@@ -0,0 +1,49 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: SyntaxVisitor.cs                                                *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using SerializationGenerator;
+
+namespace SerializationSchemaGenerator
+{
+    public class SyntaxVisitor : CSharpSyntaxWalker
+    {
+        private readonly SemanticModel _semanticModel;
+        private readonly SerializerSyntaxReceiver _syntaxReceiver;
+
+        public SyntaxVisitor(SemanticModel semanticModel, SerializerSyntaxReceiver syntaxReceiver)
+        {
+            _semanticModel = semanticModel;
+            _syntaxReceiver = syntaxReceiver;
+        }
+
+        public override void VisitClassDeclaration(ClassDeclarationSyntax node)
+        {
+            _syntaxReceiver.OnVisitSyntaxNode(node, _semanticModel);
+        }
+
+        public override void VisitFieldDeclaration(FieldDeclarationSyntax node)
+        {
+            _syntaxReceiver.OnVisitSyntaxNode(node, _semanticModel);
+        }
+
+        public override void VisitPropertyDeclaration(PropertyDeclarationSyntax node)
+        {
+            _syntaxReceiver.OnVisitSyntaxNode(node, _semanticModel);
+        }
+    }
+}