Use a config schema instead of configDefaults.
Use a config schema instead of configDefaults.

Kills a Deprecation Cop warning.

configDefaults has been deprecated since v0.133.0:
https://github.com/atom/atom/commit/f57dbfd9f531c847f8e1ce34beab2b526f71
554c

davidlormor/emmet-atom,emmetio/emmet-atom,alexlecco/emmet-atom-simplified,mat-b-/emmet-atom-simplified,davidlormor/emmet-atom,kebot/emmet-atom,mat-b-/emmet-atom-simplified,kebot/emmet-atom,kebot/emmet-atom,rkmax/emmet-atom,alexlecco/emmet-atom-simplified,davidlormor/emmet-atom,rkmax/emmet-atom,mat-b-/emmet-atom-simplified,rkmax/emmet-atom,alexlecco/emmet-atom-simplified