Better UX in CLI: "start" subcommand should be split into "start" and "create"

This might be related to #242.

I've noticed this when I set up Colima on my M1 MacBook. I ran `colima start` and it created a VM but with the `aarch64` architecture (I was unaware of that at the time). Then I tried running a container with an image produced only for `x86_64` which obviously failed. After some time I figured out that I need to create a VM with the `x86_64` architecture. I tried `colima start --arch x86_64` but the container kept failing to start. And only then I noticed that `--arch x86_64` had no effect because it was using the initially created `aarch64` VM. So, I had to delete that one first and recreate one for `x86_64`. All very confusing in terms of UX.

Considering that you _can_ have multiple VM profiles I think there should be a separate `create` subcommand which fails if a default profile already exists or overrides it with a warning, and the `start` subcommand should probably require a profile name.