Exclude npm package.json from NuGet package

By updating the `package.json` and `package-lock.json` files to `Pack=false`, we ensure they aren't copied to the NuGet package unnecessarily. These are _exclusively_ needed for the build process itself.

The SDK-style project will automatically add these as `<Content />` items _after_ the `Directory.Build.props` have run. As such, if we include them there, then they'll just get overwritten with the default configuration. If we update them in the `Directory.Build.targets`, however, they'll be correctly updated, since this is evaluated at the _end_ of the SDK-style project file.

Author: JeremyCaney

Directory.Build.targets

@@ -0,0 +1,8 @@
+<Project>
+
+  <ItemGroup>
+    <Content Update="package.json" Pack="false" />
+    <Content Update="package-lock.json" Pack="false" />
+  </ItemGroup>
+
+</Project>

OnTopic.Editor.AspNetCore.sln

@@ -11,6 +11,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 	ProjectSection(SolutionItems) = preProject
 		.gitignore = .gitignore
 		Directory.Build.props = Directory.Build.props
+		Directory.Build.targets = Directory.Build.targets
 		GitVersion.yml = GitVersion.yml
 		Icon.png = Icon.png
 		README.md = README.md