clarinet devnet start out of clarinet context

[lgalabru]

It'd be great to be able to spin up a devnet network locally without requiring a clarinet project.
We could introduce something like:

$ clarinet devnet start --default-settings

• if a Clarinet.toml manifest file can not be found in the current directory, the --default-settings mode kicks in by default (even if not provided)
• if a Clarinet.toml manifest file can be found and the flag --default-settings is provided, the manifest file is ignored.