It feels more nautral to type when doing `--launch x` or `--create x`.

Emaultor keeps running on a seuccessful launch, so this automatically returns after 3 seconds if the process hasn't quit (we have to wait for some period to get stderr in the case of a failure).

I can't come up with a better name; anything with Simulator or Device in it will be confused with the existing IOSSimulator/Device classes (which represent the running devices).

`flutter emulators nexus` will now return only those that'd match (eg. same things that `flutter emulators --start nexus` would match).

And it's not obvious to the use what the ID is.

Added an exists check to ensure we don't try to run if emulator is missing, but that requires the file extension for Windows.

Windows sets HOMEDRIVE/HOMEPATH.

It's too fast and leaves weird output on the screen