Use Theo for design token handling

[HerrBertling]

Currently, rendering design tokens works only with *.scss files (=> https://github.com/sumcumo/nuxt-styleguide/blob/master/packages/nuxt-styleguide/src/loaders/design-token-loader.js#L107 )

I'd like to change this and use Theo for generating design tokens. Things I like about this:

• Having the tokens in JSON format
• No AST parsing necessary – this choked with using !default for SCSS tokens which was mildly annoying already
• Win: Ability to output the tokens in several formats rather easily