Handle proper fallbacks in config

[RobinMalfait]

This PR will normalize the config from the old structures to the new structure. If we detect that anything is old/incorrect, we are still able to normalize it but we will also warn in the console.

We will have to improve the warning to include a link to a discussion / website to talk about the new structure.
If you happen to see this PR, here is the new structure:

module.exports = {
  content: { // Or content: ['/path-to-files', '/path-to-files-with-globs/**.js', { raw: 'text-center' }]
    files: [/* Same as above... */],
    extract: { // Or a function immediately for the DEFAULT case
      DEFAULT: (content) => ['list', 'of', 'candidates']
      html: (content) => ['list', 'of', 'candidates']
    },
    transform: { // Or a function immediately for the DEFAULT case
      DEFAULT: (content) => content,
      md: (content) => parseMarkdown(content)
    }
  },
  safelist: [
    'mt-20', // Raw strings
    { pattern: /bg-red/, variants: ['hover', 'focus'] }
  ]
}

---

TODO:

• upgrade existing tests to "new" syntax
• add a "old" tailwind config to make sure that everything still works in v3 + warning