Set PackageId to GitHub.VisualStudio.16

GitHub.VisualStudio.16.json will be created for vsinstaller build.

Author: jcansdale

src/GitHub.VisualStudio.16/source.extension.vsixmanifest

@@ -1,9 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <PackageManifest Version="2.0.0" xmlns="http://schemas.microsoft.com/developer/vsx-schema/2011" xmlns:d="http://schemas.microsoft.com/developer/vsx-schema-design/2011">
     <Metadata>
-        <Identity Id="GitHubCore.f4fbb70d-dee2-4dee-b322-bd74e3010e8f" Version="1.0" Language="en-US" Publisher="Jamie Cansdale" />
-        <DisplayName>GitHubCore</DisplayName>
-        <Description>Empty VSIX Project.</Description>
+        <Identity Id="GitHubCore.f4fbb70d-dee2-4dee-b322-bd74e3010e8f" Version="1.0" Language="en-US" Publisher="GitHub, Inc" />
+        <DisplayName>GitHub Extension for Visual Studio</DisplayName>
+        <Description>TODO</Description>
+        <PackageId>GitHub.VisualStudio.16</PackageId>
     </Metadata>
     <Installation>
         <InstallationTarget Id="Microsoft.VisualStudio.Community" Version="[16.0, 17.0)" />