Border controls: Add the ability to choose specific sides the border should appear on

[kjellr]

The current border controls do not allow us to (for example) display the border only on the top or bottom of a block. Take this Twenty Twenty-One pullquote for example:

If we apply border settings in theme.json, it'll apply the border on all sides, and there's no way to change that:

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Current border controls UI, for reference:

[skorasaurus]

This has been implemented.