Add Sustainable Web Design digital carbon rating scale

[fershad]

Is your feature request related to a problem? Please describe.
The Sustainable Web Design group have published a rating scale that aims to provide a standardised, repeatable, understandable means for sustainability ratings to be attributed to websites. This scale is based on the Sustainable Web Design methodology, as well as data from HTTP Archive.

https://sustainablewebdesign.org/digital-carbon-ratings/

Describe the solution you'd like
Provide an option for developers to return carbon estimates using the Sustainable Web Design Model which includes an equivalent rating as well. The rating system is based on kilobytes transferred, and so can use the input of bytes that is passed into either the perByte, perVisit, perByteTrace, or perVisitTrace functions to return a result.

Describe alternatives you've considered
Until now developers using CO2.js who wanted to implement the rating scale would need to code this logic themselves.

Additional context
Add any other context or screenshots about the feature request here.