fix(presets): export interfaces

[betlgtu]

Motivation and Context

Configuration options should export.

How Has This Been Tested?

visually (Intelli Sense working)

import Vue from 'vue';
import Vuetify from 'vuetify/lib';
import { UserVuetifyPreset } from 'vuetify';

Vue.use(Vuetify);

const opts: Partial<UserVuetifyPreset> = {
  theme: {
    dark: false,
  },
};

export default new Vuetify(opts);

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)