feat: allow specifying pad when using @vue/compiler-sfc

[andrewmackrodt]

Adds the ability to specify the pad mode when calling parse in @vue/compiler-sfc. This is required for accurate source maps otherwise templates are compiled without the leading new lines until <script> is encountered.

Closes #1778

Defaults to the value of sourceMaps (i.e. true | false) if unspecified.

Usage:

{
  test: /\.vue$/,
  loader: 'vue-loader',
  options: {
    pad: true, // boolean | 'line' | 'space'
  },
},