Try to render flex layout styles as presets

[tellthemachines]

What?

Tries to convert the flex layout styles into presets.

This is very much a WIP. It only addresses server rendered blocks for now, and it ignores properties for which arbitrary values can be set (e.g. gap).

Why?

Inspired by #36944 and #39374, mainly because I was wondering what a default approach would look like for layout.

How?

• Flattens out the styles needed for flex layout so they fit neatly into classname -> property pairs.
• Uses readable, verbose classnames for each property, because Classnames are Poetry (or something).

Testing Instructions

1. Pick a server rendered block with flex layout that is NOT the Navigation block (because that one has custom logic for its styles). Query Pagination seems to be the only core one available.
2. Check that changing its layout works as expected on the front end.

Screenshots or screencast

[tellthemachines]

Closing this now that we've landed #40875 🎉