Previously, `Main.run` considered the first item in `argv` array a command ID, for example, in the `['user:add', 'Peter']` array, the command was `user:add` and the rest were its args. This doesn't work with space-separated commands which produce `argv` array like this:

```
['user', 'add', 'Peter']
```

and would be parsed as a `user` command that gets `add` and `Peter` as args.

The new implementation looks at `config.commandIDs` and tries to find the longest match with items from `argv`. For example, if `commandIDs` contained `user` and `user add` commands, the new implementation would invoke a `user add` command with `['Peter']` as args. If, on the other hand, `commandIds` only contained the `user` command, it would invoke a `user` command with `['add', 'Peter']` as args.

A couple of notes:

- It's up to @oclif/config (or possibly a plugin) to produce command IDs that contain spaces. As of @oclif/[email protected], the command IDs contain a colon, e.g., `user:add`.
- The change is backwards compatible. Argv `['user:add', 'Peter']` is still parsed as command ID `user:add` and `['Peter']` as args.
- The command separator is currently hardcoded to be a space. If oclif introduced a new config option like `"separator": "<any_character>"`, the argv splitting logic should be updated.
- The `splitArgv` function should be unit-tested. I didn't find a testing infrastructure in this package, will seek advice on how to best do it.