dispose fsi at the end of a scripting session (dotnet#8084)

Author: brettfo

src/fsharp/FSharp.Compiler.Private.Scripting/FSharpScript.fs

@@ -83,3 +83,4 @@ type FSharpScript(?captureInput: bool, ?captureOutput: bool, ?additionalArgs: st
             stdin.Dispose()
             stdout.Dispose()
             stderr.Dispose()
+            (fsi :> IDisposable).Dispose()