Add Prerequisites to GitHub.StartPage .vsixmanifest

Author: jcansdale

src/GitHub.StartPage/source.extension.vsixmanifest

@@ -15,4 +15,7 @@
   <Assets>
     <Asset Type="Microsoft.VisualStudio.VsPackage" d:Source="Project" d:ProjectName="%CurrentProject%" Path="|%CurrentProject%;PkgdefProjectOutputGroup|" />
   </Assets>
+  <Prerequisites>
+    <Prerequisite Id="Microsoft.VisualStudio.Component.CoreEditor" Version="[15.0.25824.0,16.0)" DisplayName="Visual Studio core editor" />
+  </Prerequisites>
 </PackageManifest>