Fix merging of arrays during config resolution

[bradlc]

Fixes #9666

The result of the theme values in the extend section are not deeply merged when they are simple arrays test prior to this fix (the new array is appended onto the existing array instead of overriding it):

{
  fonts: {
    sans: ['Comic Sans'],
    serif: ['Papyrus', { fontFeatureSettings: '"cv11"' }],
    mono: [
      ['Menlo', 'Courier New', 'monospace'],
      ['Lobster', 'Papyrus'],
      { fontFeatureSettings: '"cv11"' },
    ],
  },
}

This fix assumes that the original intent was to use isPlainObject instead of the isObject function that returns true for arrays.