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/SourceCodeAnalysis.cs

@@ -0,0 +1,49 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: SourceCodeAnalysis.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 System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using Microsoft.Build.Locator;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.MSBuild;
+
+namespace SerializationSchemaGenerator
+{
+    public static class SourceCodeAnalysis
+    {
+        public static List<(Project, Compilation)> GetCompilation(string solutionPath)
+        {
+            if (!File.Exists(solutionPath) || !solutionPath.EndsWith(".sln", StringComparison.Ordinal))
+            {
+                throw new FileNotFoundException($"Could not open a valid solution at location {solutionPath}");
+            }
+
+            MSBuildLocator.RegisterDefaults();
+
+            var workspace = MSBuildWorkspace.Create();
+
+            var solutionToAnalyze = workspace.OpenSolutionAsync(solutionPath).Result;
+
+            var results = solutionToAnalyze.Projects.AsParallel()
+                .Select((project) => (project, project?.GetCompilationAsync().Result))
+                .Where((value) => value.Result != null)
+                .ToList();
+
+            return results;
+        }
+    }
+}