feat: add hooks in generate mode before and after file generation

[patricrichard]

Why is the change needed?

The current implementation only resolves the options used to generate the robots.txt statically from nuxt.config.js. This limits the module's use cases as it is not possible to extend the config from another nuxt module during static site generation.

How is the need addressed?

By using the same pattern as @nuxtjs/sitemap do for this feature

• Calling robots:generate:before hook passing options before file generation
• Calling robots:generate:done hook after file generation

What side-effects could the change have?

• The options can be modified by other nuxt modules during generation

[ricardogobbosouza]

Thanks @emma-patriclundin