add core editor prerequisite

Beckman.Snippets/source.extension.vsixmanifest

 <?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="Beckman.Snippets.Andrew Reynolds.554a17d5-acf8-469c-ad75-060f99bab879" Version="1.2.3" Language="en-US" Publisher="Andrew Reynolds" />
-    <DisplayName>Beckman Snippets</DisplayName>
-    <Description xml:space="preserve">Visual Studio code snippets commonly used in Beckman projects.</Description>
-    <MoreInfo>http://madskristensen.net/post/shipping-visual-studio-snippets-in-an-extension</MoreInfo>
-    <License>License.txt</License>
-    <ReleaseNotes>ReleaseNotes.txt</ReleaseNotes>
-    <Tags>snippets</Tags>
-  </Metadata>
-  <Installation>
-    <InstallationTarget Id="Microsoft.VisualStudio.Community" Version="[14.0,)" />
-  </Installation>
-  <Dependencies>
-    <Dependency Id="Microsoft.Framework.NDP" DisplayName="Microsoft .NET Framework" d:Source="Manual" Version="[4.5,)" />
-  </Dependencies>
-  <Assets>
-    <Asset Type="Microsoft.VisualStudio.VsPackage" d:Source="File" Path="BeckmanSnippets.pkgdef" />
-  </Assets>
+    <Metadata>
+        <Identity Id="Beckman.Snippets.Andrew Reynolds.554a17d5-acf8-469c-ad75-060f99bab879" Version="1.2.3" Language="en-US" Publisher="Andrew Reynolds" />
+        <DisplayName>Beckman Snippets</DisplayName>
+        <Description xml:space="preserve">Visual Studio code snippets commonly used in Beckman projects.</Description>
+        <MoreInfo>http://madskristensen.net/post/shipping-visual-studio-snippets-in-an-extension</MoreInfo>
+        <License>License.txt</License>
+        <ReleaseNotes>ReleaseNotes.txt</ReleaseNotes>
+        <Tags>snippets</Tags>
+    </Metadata>
+    <Installation>
+        <InstallationTarget Id="Microsoft.VisualStudio.Community" Version="[14.0,)" />
+    </Installation>
+    <Dependencies>
+        <Dependency Id="Microsoft.Framework.NDP" DisplayName="Microsoft .NET Framework" d:Source="Manual" Version="[4.5,)" />
+    </Dependencies>
+    <Assets>
+        <Asset Type="Microsoft.VisualStudio.VsPackage" d:Source="File" Path="BeckmanSnippets.pkgdef" />
+    </Assets>
+    <Prerequisites>
+        <Prerequisite Id="Microsoft.VisualStudio.Component.CoreEditor" Version="[15.8.27729.1,16.0)" DisplayName="Visual Studio core editor" />
+    </Prerequisites>
 </PackageManifest>