Build FSI and FSC using AppHost

[KevinRansom]

Since the begining we have built the coreclr versions of fsc and fsi as il .exe files. The AppHost wasn't introduced at the beginning, and we were in a groove that worked for us.

This changes things so that we build them as fsi.dll and fsc.dll and produce apphost fsi.exe and fsc.exe.

this means that things like this work:

C:\Users\codec>c:\kevinransom\fsharp\artifacts\bin\fsi\Release\netcoreapp3.1\fsi.exe

Microsoft (R) F# Interactive version 11.0.0.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

>  printfn "%s" typeof<int>.Assembly.Location;;
C:\Program Files\dotnet\shared\Microsoft.NETCore.App\3.1.10\System.Private.CoreLib.dll
val it : unit = ()

> #quit;;

C:\Users\codec>c:\kevinransom\fsharp\artifacts\bin\fsc\release\netcoreapp3.1\fsc.exe
Microsoft (R) F# Compiler version 11.0.0.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

error FS0207: No inputs specified

C:\Users\codec>

It should be noted that even after this change, the only supported scenarios for using fsi is via dotnet fsi and fsc is via dotnet build. The apphost versions will make things a little easier in the repo, and tooling may be a bit easier to write.

[dsyme]

Something is causing the proto compiler to be continually rebuilt on every

 .\build

when the compiler files have changed. Could it be this or another recent PR?