Add a check that 'default' is the last option specified

[mattpocock]

WRONG:

{
  "default": "./2.js",
  "require": "./1.js"
}

RIGHT:

{
  "require": "./1.js",
  "default": "./2.js"
}

Next.js (and I assume, Webpack 5) errors if you don't get this right.

[XantreDev]

marvinhagemeister if i I'll provide PR can you review it?