Segmented control styles are not correct in Storybook

[mperrotti]

Describe the bug

1. :hover and :focus styles not being shown
2. Dividers are not being shown between buttons

To Reproduce
Steps to reproduce the behavior:

1. Go to https://primer.style/css/storybook/?path=/story/components-segmentedcontrol--basic or any other segmented control story
2. See that the styles are not showing as expected

Expected behavior

Screenshots

Desktop (please complete the following information):

• OS: macOS
• Browser Chrome
• Version 104

Additional context
This is not a bug with the SCSS, the Storybook markup just needs to be updated. These issues were likely caused when we updated segmented controls to be an unordered list in this PR.

The hover and active styles should start being shown once we set aria-current={"true"|"false"} on each button

The dividers should start showing once we scope the divider styles to the list items instead of the buttons. Currently, we use an adjacent sibling selector on the button class to show these styles: src/segmented-control/segmented-control.scss#L58