Improve performance of get_block_theme_folders

[abhansnuk]

Trac ticket: https://core.trac.wordpress.org/ticket/58319

Owner: spacedmonkey

Jamie to check if it is covered in the three dev note drafts already received. This is one of the dev notes being wrangled by performance, so can check if it is on their wrangling ticket.

[github-actions]

Heads up @WordPress/docs-issues-coordinators, we have a new issue open. Time to use 'em labels.

[codente]

This is covered in the performance dev note: Improves to template loading in WordPress 6.4
These have been drafted by Jonny Harris, and waiting for review in performance and then review in docs.
https://wordpress.slack.com/archives/C02KGN5K076/p1697201832774819

Google doc https://docs.google.com/document/d/1avt7Yh_PArpD2NcUVKYxd4yUkTwDCGNCVhp6LmR1VUM/edit

[nalininsbs]

As certain fields are not available to the docs release group, an additional note providing an overview of performance tickets in the run up to the Field Guide deadline, can be found on this docs sheet: Google sheet to better understand status on the performance labelled tickets. Note performance team is wrangling those tickets for identification, first draft and performance first review.

[nalininsbs]

Part of https://make.wordpress.org/core/2023/10/17/improvements-to-template-loading-in-wordpress-6-4/