fix(types): class names are not all mandatory

[cexbrayat]

🔎 Overview
This PR updates the type of ClassNames to make every field optional.

🤓 Code snippets/examples (if applicable)

AFAIK, it is possible to only declare:

Vue.use(VeeValidate, {
  classes: true,
  classNames: {
    invalid: 'is-invalid',
  },
});

without having to declare every classes.
The current typings don't allow that.
The PR makes every class optional, making such a configuration possible.

[codecov]

Codecov Report

Merging #2108 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2108   +/-   ##
=======================================
  Coverage   95.86%   95.86%           
=======================================
  Files          62       62           
  Lines        2369     2369           
  Branches      743      743           
=======================================
  Hits         2271     2271           
  Misses         89       89           
  Partials        9        9

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[logaretm]

Great, thanks for the correction and much appreciated!