Generate _headers file

[calavera]

Is your feature request related to a problem? Please describe.

Assets under /_next/static/chunks are always immutable. They can be cached by the browser forever, without having to re-validate their content. Netlify's default cache-control header is not optimal for those assets, however, it can be overriden with a _headers file.

Describe the solution you'd like

Add a new step that generates a _headers file and overrides the cache control for /_next/static/chunks. This can be acomplished with this rule:

/_next/static/chunks/*:
  cache-control: public,max-age=31536000,immutable

See the reference in our docs: https://docs.netlify.com/routing/headers/#syntax-for-the-netlify-configuration-file

Describe alternatives you've considered

We can leave it as it is.