V13: Add rcl files to static files tree

[Zeegaan]

Fixes #15956

Notes

• This refactors the StaticFilesTreeController to now use IWebhostEnvironment to get wwwroot files, this will then also get files from RCLs.
• Some logic to now go over:
• If path is empty, we assume we are "just" asking for the root folders "wwwroot" & appplugins,
  This was the business logic before, and was filtered by this code if (id == Constants.System.RootString && directory != AppPlugins && directory != Webroot)
• If we want files from the AppPlugins director, we still want to use the IPhysicalFileSystem, so the behavior will remain, as you cannot put RCL files in there. (It has to be in wwwroot)
• If we want files from wwwroot folder, then we now use IWebHostEnvironment.WebRootFileProvider, but the path is a little weird here, so we have to recalculate, as when you pass in "" in the .GetDirectories, this will assume you are already in /wwwroot
• This logic should probably be in a service, and will be refactored for 14+ to be in a service ala something like StaticFilesService

How to test

• Follow steps in original issue.
• When creating a RCL with HTML file in the wwwroot folder, here is my csproj from my test project:

<Project Sdk="Microsoft.NET.Sdk.Razor">
  <PropertyGroup>
    <Title>TestProj</Title>
    <Description>Adds support for Entity Framework Core to Umbraco CMS.</Description>
    <EnablePackageValidation>false</EnablePackageValidation>
    <AddRazorSupportForMvc>true</AddRazorSupportForMvc>
    <StaticWebAssetBasePath>/</StaticWebAssetBasePath>
  </PropertyGroup>
</Project>

[kjac]

Works like a charm! 🚀

[Zeegaan]

Cherry picked for 13.6 🚀