Button Block: use computed style to set default border-radius

[stokesman]

Taking a stab at #17596, this employs getComputedStyle to define the default border-radius for buttons.

In a theme with square buttons:

Sorry about the weird artifacts on the bottom of these screen captures. Hopefully, they're not too distracting.

In a theme defining a border-radius larger than the current built-in maximum (and in units other than pixels thus the floating number):

It could be set up so that the maximum border-radius is raised by the computed style but not sure that's desired.

This works well with any theme I've tested. I think an issue could arise if a theme/plugin defines style variations that are supposed to have a different border-radius. I've not tested it but if I've done my thinking correctly they would show in-canvas with the intended border-radius but the range control would read as if they have the computed radius. I think that could be worked around too but at the moment this is mainly aimed at sparking conversation.

How has this been tested?

Tested in
Wordpress 5.5.1

Types of changes

Non-breaking enhancement

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• [n/a] My code follows the accessibility standards.
• My code has proper inline documentation.
• [n/a] I've included developer documentation if appropriate.
• [?] I've updated all React Native files affected by any refactorings/renamings in this PR.

[stokesman]

Closing in favor of #29193