Add feature for Proofreader API

[reillyeon]

Specification

https://github.com/webmachinelearning/proofreader-api/blob/main/README.md (spec link coming, there's a GitHub issue)

Description

The Proofreader API uses generative AI to look for and correct errors in grammar, spelling, and punctuation.

Documentation

https://developer.chrome.com/docs/ai/proofreader-api

Browser support

Active Origin Trials in Google Chrome and Microsoft Edge.