Add project files.

Assets/Editor/FixProjectVersion.cs

@@ -0,0 +1,15 @@
+#if UNITY_5
+using System.Text.RegularExpressions;
+using SyntaxTree.VisualStudio.Unity.Bridge;
+using UnityEditor;
+
+[InitializeOnLoad]
+public class ProjectFilesGeneration {
+	static ProjectFilesGeneration() {
+		ProjectFilesGenerator.ProjectFileGeneration += ReplaceContent;
+	}
+	static string ReplaceContent(string n, string c) {
+		return Regex.Replace(c, @"<LangVersion(.*?)>.*?</LangVersion>", @"<LangVersion$1>3</LangVersion>");
+	}
+}
+#endif

Assets/Editor/FixProjectVersion.cs.meta

@@ -0,0 +1,12 @@
+fileFormatVersion: 2
+guid: 62489f8e495a805478e5b45c0f53ca4e
+timeCreated: 1623583546
+licenseType: Pro
+MonoImporter:
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: