Move Blazor to .NET 5

[pranavkm]

Runtime and tools story

The .NET WebAssembly team is driving the work for a build pipeline for a WASM. Once we have designs for it available, we'll have to figure out how blazor fits in to it. Partner work is TBD at this point

Detecting a blazor-wasm project

The SDK will need some way of knowing if a project is a Blazor WASM application to enable BlazorWASM specific targets of the build pipleline. One option might be to rely on a WASM specific rid if that's available. A better option would be to use an explicit property. Mvc applications do this (AddRazorSupportForMvc) and so do WinForms applications UseWindowsForms so there's incentive to be consistent:

<PropertyGroup>
    <TargetFramework>net5</TargetFramework>
    <UseBlazorWebAssembly>true</UseBlazorWebAssembly>
  </PropertyGroup>

Sharing the linker

With the compiler (csc), we re-use it tasks, but not it's targets. There's additional overhead in making sure these targets are kept up to date with the task, but we've shipped targets for CSC this way in RazorSDK for a while now, and there isn't an alternative that is clearly better.

Dev-Server, DebugProxy

Tools that ship as part of the WebSDK, similar to dev-certs, user secrets etc.