Add check for `implies` argument type

[canopy-js-user]

Hi there, twice I've accidentally written a line of code like this:

.addOption(new Option(
    '--port <number>', 
    'Which port to run the server on for sync mode'
).default(undefined).implies('sync'))

Ie, I want the --port option to imply the boolean flag sync. However, sync is expecting an object of implied key-value pairs. However, as is, this fails silently and converts the string/array 'sync' into an object, and merges it with the rest of the options:

{
  '0': 's',
  '1': 'y',
  '2': 'n',
  '3': 'c',
  editor: true,
  logging: true,
  open: true,
  backup: true,
  sync: true,
  port: '4004'
}

I'm proposing a check that the argument to implies is an object and not a string, and if it is a string, error. (Or, interpret the string as the name of a boolean option that the programmer is asserting should be true.)

Thanks!

[canopy-js-user]

(Another reason I think this an easy mistake is that many of the option methods do take strings, eg .conflicts. Thanks!)

[shadowspawn]

Commander does not have a lot of runtime checks on the parameters, but I am happy to consider adding checks or improving behaviour for routines that are easy to get wrong.

(A past one that was added was #1655.)

[canopy-js-user]

Makes sense, thanks!

[shadowspawn]

For interest, we added support in .conflicts() for a string or an array after making a similar mistake during development: #1678 (comment)

[canopy-js-user]

@shadowspawn Very nice!

[shadowspawn]

Shipped #1854 in 10.0.1

Thanks @canopy-js-user

[canopy-js-user]

@shadowspawn thank you!