Implement MapPath and AppDomainAppPath

[CZEMacLeod]

Implement HttpRuntime.AppDomainAppPath, HttpServerUtility.MapPath, and HttpServerUtilityWrapper.MapPath

Provide support for MapPath
Paths are mapped relative the application root directory - available as HttpRuntime.AppDomainAppPath

This may not behave as expected, for example if a person uses MapPath to write uploaded files to disk and expects them to be served, due to the way AspNetCore uses ContentRootPath/WebRootPath.
In this case it might be better to implement your own helper function which can use something like

namespace System.Web.Extensions
{
    public static class MapPathExtension
    {
        public static string MapContentPath(this System.Web.HttpContext context, string? path)
        {
#if NET6_0_OR_GREATER
            var appPath = path is null ? VirtualPathUtility.GetDirectory(context.Request.Path) :
                VirtualPathUtility.Combine(
                VirtualPathUtility.GetDirectory(context.Request.Path) ?? "/"
                , path);
            var coreContext = (Microsoft.AspNetCore.Http.HttpContext)context;
            var rootPath = coreContext.RequestServices.GetRequiredService<IWebHostEnvironment>().WebRootPath;
            if (string.IsNullOrEmpty(appPath)) return rootPath;
            return System.IO.Path.Combine(rootPath, appPath[1..].Replace('/', System.IO.Path.DirectorySeparatorChar));
#else
            return context.Server.MapPath(path);
#endif
        }
    }
}

Addresses #183

[twsouthwick]

@CZEMacLeod this builds on #179, right? If so, I'll hold off on reviewing until we get that in.

[CZEMacLeod]

@twsouthwick Yes - It uses the VirtualPathUtility from #179 to handle the app relative and request relative paths and prevent 'exiting' up above the site directory.

[CZEMacLeod]

@twsouthwick The tests I've added seem to be failing on Linux/Mac - I think because the expected results rely on the directory separator character but I'm not sure how to find the output, or what to do to mitigate this...

[twsouthwick]

Can you clean up the comments and such in the MapPath implementation to just have the direction we decided on?

[twsouthwick]

Thanks @CZEMacLeod for the contribution! LGTM