Remove copied static files code from WebView project

[Eilon]

As a workaround for another problem we introduced a lot of copied code into the Blazor Desktop WebView package: #31071

There's a fair bit of copied code related to static files and content types. The Blazor Desktop packages can't depend on anything in Microsoft.AspNetCore.App because they have to run in places where ASP.NET Core doesn't run, such as Android and iOS.

Options:
0. Do nothing and keep the copied code (possibly trimming it down to the minimal amount needed, rather than taking entire files)

1. Move the code to a NuGet package that can be used from everywhere
2. Something else?

[Eilon]

Ideas:

1. Delete the copied code that does file extension mapping
2. Instead, have an MSBuild-time target that computes the MIME types using an MSBuild item group that has all the MIME mappings
   • This would enable users to update/add/remove MIME types if they need something custom
3. The MSBuild target would store the computed MIME types in some new asset
4. In the WebViewManager code load the computed MIME types along with the list of embedded files and serve it up

We still need to figure out what to do with copied code for the static file provider that is itself copied, plus the path/query string code that is copied.