fix(transformer-directives): enforce pre by default

[sibbng]

Fixes #1849

In production builds Nuxt's cssnano plugin applied before unocss plugin. Its mergeRules optimization causes problems with at-rules. My previous solution was disabling mergeRules by adding the following lines to @unocss/nuxt module. But that's not ideal since it affects minifying:

    if (nuxt.options.postcss.plugins.cssnano) {
      const preset = nuxt.options.postcss.plugins.cssnano.preset
      nuxt.options.postcss.plugins.cssnano = {
        preset: [preset?.[0] || 'default', Object.assign(
          preset?.[1] || {}, { mergeRules: false },
        )],
      }
    }

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