Buttons: Enable Fit / Grow / Fixed width controls using layout support

[sarthaknagoshe2002]

What?

Closes #76531

Why?

Currently, Button blocks inside a Buttons block do not expose the same sizing controls (Fit / Grow / Fixed) that are available when placed inside a Row block.

As a result users cannot proportionally size multiple buttons.

How?

The PR sets "allowSizingOnChildren" to true & removes existing width control.

Testing Instructions

1. Open a post or page in the editor
2. Insert a Buttons block
3. Add two or more Button blocks inside it
4. Select one of the Button blocks
5. Open the Settings sidebar

Screenshots or screencast

Before	After

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[t-hamano]

Thanks for the PR!

If my understanding is correct, I don't think we can simply replace width support with layout support because it would break backward compatibility. It might be possible to migrate to layout support, but that would likely require a fairly complex migration process.

cc @aaronrobertshaw @ramonjd @andrewserong

[ramonjd]

I think @t-hamano is right: removing a block support would cause regressions.

Here's what happens with existing block markup containing button widths:

Also the control isn't set up to support theme presets, which was one of the defining motivations behind #74242 if I've not mistaken:

Not saying that harmonizing layout and width is impossible, but it requires a little more groundwork.