Boost: Improve critical css generation process (#41946)

* Add watch command

* Check http code before proceeding with C.CSS generation

We need to do this, otherwise the library will generate css for
the 404 page.

* Update css generation to catch invalid URL errors and surface them

* Update recommendations to be split by type as well as provider key

WIP

* Add changelogs

* Refactor recommendations to allow dismissing unique provider errors

* Update URL validation

* Update helper description

* Add proper troubleshooting steps for invalid URLs

* Update status code validation

* Fix tests

* Throw an error on invalid url

Co-authored-by: Liam Sarsfield <[email protected]>

* Remove redundant fragment

Co-authored-by: Liam Sarsfield <[email protected]>

* Improve type safety and readability of error grouping

* Update step

Co-authored-by: Adnan Haque <[email protected]>

* Add an additional step

* Update http status check to check for a 404 instead

* Revert slashes to see if opaque redirects work

* Simplify 404 check

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Co-authored-by: Liam Sarsfield <[email protected]>
Co-authored-by: Adnan Haque <[email protected]>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/13675171752

Upstream-Ref: Automattic/jetpack@8a4e01c

Author: dilirity