fix(btn): improve focus style to meet WCAG 2.2 AA

[dancormier]

Note The PR supersedes #1610 and all commits here are cherry-picked from that PR with the exception of the visual regression image generation. @CGuindon and @giamir I added you as reviewers for visibility.

Addresses STACKS-537, partially addresses A11Y-2.

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This PR updates the focus styles for the button component to meet the WCAG AA requirements.

Technical notes

You'll notice that this PR achieves the focus style mainly with box-shadow:

   box-shadow: inset 0 0 0 var(--su-static2) var(--theme-secondary-400), inset 0 0 0 var(--su-static4) var(--black-050);
    …
   outline: var(--su-static2) solid transparent;

This is because box-shadow tends to align more consistently. Additionally, a transparent outline is used to ensure that an outline doesn't render when expected but can still be rendered in forced-color modes (see https://blogs.windows.com/msedgedev/2020/09/17/styling-for-windows-high-contrast-with-new-standards-for-forced-colors/#gist105426765).

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