Editor formatting process should continue running in the background

[dsherret]

C# editor formatting is too slow—around 500ms on my machine. This is because C# is slow on initial startup (due to JIT or perhaps the compiler is initializing some things).

At the moment, a cli process is created on every file format, but it would be better to have it run once then communicate with it as it’s running. This should get formatting c# files down to under 50ms.