Allows the Visual Studio Settings Designer to read from and write to Web.config (#97)

# Summary
Closes #95 - where opening or using the Settings Designer in Visual Studio creates a new `App.config` and writes settings there, instead of using `Web.config`.

# What this change does
When `App.config` does not exist, we link `Web.config` as `App.config` (hidden & not part of the project) so the designer "finds" an `App.config`, but actually reads/writes `Web.config`.

# Rationale
The .NET Settings Designer expects `App.config`. In Web projects that only have `Web.config`, the designer creates a new `App.config` and starts using it, which is undesirable for ASP.NET. By linking `Web.config` as `App.config` (keeping it out of Solution Explorer to avoid clutter), the designer stays happy and all settings end up in Web.config, without leaving behind a stray `App.config`.

# Behavior & edge cases
- If `App.config` already exists: nothing changes; we do not add the linked item
- If `Web.config` is missing: nothing changes; no link is added

#Manual verification steps
- Create an SDK-style ASP.NET 4.x project with only Web.config (no App.config) - e.g. dotnet new -i systemwebfull
- Open in Visual Studio -> Project Properties -> Settings -> "Create or open application settings"
- Add a new setting
- Expected: VS Settings Designer writes to `Web.config`; no `App.config` file is created on disk or added to the project
- Add a real `App.config` and repeat
- Expected: the SDK does not add the link, designer uses the existing App.config (no behavior change from today)

#Backwards compatibility
Fully backwards compatible:

- Projects already using App.config are unaffected
- Projects with only Web.config get the desired designer behavior

Author: alatanza

src/MSBuild.SDK.SystemWeb/Sdk/MSBuild.SDK.SystemWeb.DefaultItems.props

@@ -31,6 +31,15 @@
     https://github.com/CZEMacLeod/MSBuild.SDK.SystemWeb/issues/43 
     -->
     <Content Include="Web.*.config" Exclude="@(None)" />
+
+    <!-- Allows the Visual Studio Settings Designer to read from and write to Web.config
+    https://github.com/CZEMacLeod/MSBuild.SDK.SystemWeb/issues/95
+    -->
+    <None Include="Web.config" Condition="EXISTS('Web.config') AND !EXISTS('App.config')">
+      <Link>App.config</Link>
+      <Visible>false</Visible>
+      <InProject>false</InProject>
+    </None>
   </ItemGroup>
 
   <!-- Support App_GlobalResources folder