Tailwind CSS multiple file extension content pattern

[cossssmin]

This PR fixes an issue with content patterns for Tailwind CSS when specifying multiple file extensions for Maizzle templates.

Basically if you had something like this:

module.exports = {
  build: {
    templates: {
      filetypes: ['html', 'yaml'],
      source: 'src/templates',
    }
  }
}

The automatically-generated content path was:

'src/templates/**/*.html,yaml'

While Tailwind doesn't warn about it, the pattern won't work to search in all html and yaml files at that location, resulting in some CSS not being generated. With this fix, it will be correctly generated to:

'src/templates/**/*.{html,yaml}'