Add/button block border support

[tellthemachines]

Fixes #40696

Proposed changes:

• Adds border support to the Buttons block.

WIP - perhaps should add shadow support too? It's not explicit in the original issue.

Not sure if tests are applicable for this change. Guidance appreciated! It's my first PR to this repo 😊

Also not sure which labels to add to the PR.

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Testing instructions:

• Go to '..'

[github-actions]

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[aaronrobertshaw]

The border block support also allows "per-side" or split borders e.g. border-top, border-left etc.

The style engine's get styles function will convert a style object to the appropriate CSS classes and inline styles. Here's core's version, wp_style_engine_get_styles, and Gutenberg's.

An example of its use can be found in the border block support linked above. The results are effectively the same as those on the JS side via useBorderProps.