fix: variadic arguments error parsing

[xueelf]

Although it is stated in the readme that variadic arguments can only the last parameter, there is no syntax check in the code.

Code

const cli = require('cac')()

cli
  .command('parse [a] [...b] [...c] [d]', 'Command parse')
  .action((a, b, c, d, options) => {
    console.log(a)
    console.log(b)
    console.log(c)
    console.log(d)
  })

cli.parse()

Terminal

This code still works, but it doesn't work as expected

Current

$ node index.js parse a b c d

a
['b', 'c', 'd']
['c', 'd']
d

Improved

$ node index.js parse a b c d

CommandError: only the last argument can be variadic 'b'

(｡･∀･)ﾉﾞ Will now directly throw an exception.