Add startDart to public api #5601

Add an additional startDart method to the new public api that just returns the SpawnedProcess immediately after starting it (so not piping it to an OutputChannel and not waiting until it completes).

I was previously using safeToolSpawn directly and not going through runCommandInFolder.

For starting the Jaspr debug server, I need to run dart, but manage the process myself, including listening to live stdout, writing to stdin, and manage its lifespan (close event / killing it) because its a long running process. For these reasons I can't use the current runDart api. schultek/jaspr#482