fix(host): set LangVersion=latest + Nullable=enable on VSTO csproj

The legacy VSTO csproj defaults to C# 7.3, but the host code (e.g.
AddInHost, MailItemExtractor) uses nullable reference types — which
require C# 8.0+. Building fails with ~12x
"Feature 'nullable reference types' is not available in C# 7.3".

Sync with the SDK-style Core/UI/Tests projects that already declare
LangVersion=latest + Nullable=enable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/OutlookAddin/OutlookAddin.csproj

@@ -28,6 +28,8 @@
     <AssemblyName>MarcusLaw.OutlookAddin</AssemblyName>
     <LoadBehavior>3</LoadBehavior>
     <TargetFrameworkVersion>v4.8</TargetFrameworkVersion>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
     <DefineConstants>VSTO40;UseOfficeInterop</DefineConstants>
     <BootstrapperEnabled>true</BootstrapperEnabled>
     <BootstrapperComponentsLocation>HomeSite</BootstrapperComponentsLocation>