Closes #2846

Some things to note:

• Most CLI options are no longer required, although they must be set either in json or through CLI. This could be confusing. I tried to look in to building options conditionally, so that e.g. options could be required if --json was not used or default config was not found, but this doesn't seem to be possible
• The complete config is validated, so users will still get an error if options are missing. We could probably work on making the errors more clear.
• CLI options take priority over json config. This is usually how CLI tools work.
• Because CLI options take priority over json, we need to know whether an option is supplied or not. If we specify default values in the .options(...) call, there is no way to tell a default options (e.g. no --out-dir supplied) from a supplied option with the same value (--out-dir ./design-tokens). Therefore, default options are not specified there, but moved to the section "Create final props from defaults, json file and command-line options".

I wish CommanderJS had a built-in support for config files, since the default / options / json logic got pretty complex 😢