[WIP] Post Template: Try adding flex layout and block spacing support

[andrewserong]

What?

🚧 🚧 🚧 🚧 This is an experimental WIP PR and currently does not work! 🚧 🚧 🚧 🚧

This PR seeks to explore some of the issues that will need to be resolved in order to support block spacing in the post template block. The longer-term goal is to come up with a fix for #44557

Why?

The Post Template block is a very useful and frequently used block for laying out a list of posts, and it'll likely be very beneficial to have more control over how it's displayed, along with its positioning and spacing.

How?

Try opting the Post Template block into flex layout and opt-in to block spacing

Testing Instructions

TBC

Tasks

• The Query Loop block currently passes down the ad-hoc layout type. Can we get the local layout type of the Post Template block to override this, and as a result, workaround some of the backcompat issues with introducing flex layout here?
• How can we allow switching between Flex and Flow/Constrained in an intuitive way? The layout switcher component at the moment isn't very user friendly
• How do we deal with the default 1.25em gap value, and allow themes / editors to override this value without breaking anything?
• How do we add in vertical alignment controls with a safe default?

Testing Instructions for Keyboard

Screenshots or screencast

It's pretty broken, but it's a start:

[github-actions]

Flaky tests detected in 08dabf0.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4249790365
📝 Reported issues:

• [Flaky Test] should navigate using the list view sidebar #47001 in specs/editor/various/block-hierarchy-navigation.test.js