types: add loader options declarations

[kbtz]

This PR essentially adds the interface VueLoaderOptions that can aid type checking
when writting Webpack configuration with Typescript:

// webpack.config.ts

import { VueLoaderOptions, VueLoaderPlugin } from 'vue-loader'

export default {
  module: {
    rules: {
      // ...
      {
        test: /\.vue$/,
        loader: 'vue-loader',
        options: {
          // * autocompletion available here *
        } as VueLoaderOptions
      }
    }
  },
  plugins: [
    new VueLoaderPlugin()
  ]
}

I'm studying the possibility of using a discriminated union to assign the options type when a match loader: 'vue-loader' is found, but this will come in a later PR. For now the casting on the options object is required.

---

Please note the compiler interface use types that were added only in Vue 2.6 (vuejs/vue#7918), but folks with older versions of the template compiler shouldn't experience any errors, they will just not get any type checking on compile and compileOptions properties.

[vue-bot]

Hey @kbtzr, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚