Improve accessibility of diff options button

[sergiou87]

xref. https://github.com/github/accessibility-audits/issues/4903
xref. https://github.com/github/accessibility-audits/issues/4914

Description

This PR sets the right accessibility label and aria-expanded attributes to the diff options button.

Screenshots

CleanShot.2023-07-11.at.17.37.27.mp4

CleanShot.2023-07-11.at.18.31.40.mp4

Release notes

Notes: [Improved] Improve accessibility of diff options

[tidy-dev]

I updated this PR to not use the <button> (native) title attribute because it uses the system tooltip and is inconsistent with our other button's tooltips and not accessible to keyboard users.

I instead implemented the tooltip approach from our <Button> (with a capital) react component so that the "Diff Options" would display on mouse hover and keyboard focus. (Diff options I assume doesn't use that component as we are applying some custom styling here) And, added the aria-label since the title would no longer announce.

CleanShot.2023-07-19.at.15.01.12.mp4

[0071650806]

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