refactor: correctly setup directory.*.props files [skip ci]

Author: ChristianHaase

ByteGuard.FileValidator.slnx

@@ -1,8 +1,12 @@
 <Solution>
-  <Folder Name="/Docs/">
+  <Folder Name="/assets/">
+    <File Path="Build.ps1" />
+    <File Path="Directory.Build.props" />
+    <File Path="Directory.Version.props" />
+    <File Path="LICENSE" />
     <File Path="README.md" />
   </Folder>
-  <Folder Name="/Tests/">
+  <Folder Name="/tests/">
     <Project Path="tests/ByteGuard.FileValidator.Tests.Unit/ByteGuard.FileValidator.Tests.Unit.csproj" />
   </Folder>
   <Project Path="src/ByteGuard.FileValidator/ByteGuard.FileValidator.csproj" />

Directory.Build.props

@@ -0,0 +1,15 @@
+<Project>
+  <Import Project="Directory.Version.props" />
+  <PropertyGroup>
+    <LangVersion>latest</LangVersion>
+    <TreatWarningsAsErrors>True</TreatWarningsAsErrors>
+    <CheckEolTargetFramework>false</CheckEolTargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
+    <PublishRepositoryUrl>true</PublishRepositoryUrl>
+    <EmbedUntrackedSources>true</EmbedUntrackedSources>
+    <IncludeSymbols>true</IncludeSymbols>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+  </PropertyGroup>
+</Project>