Focus outlines on Invisible & Close buttons

[emplums]

Describe the bug
Today while working on adding the CloseButton into Primer React Components, I noticed that the example in Primer CSS for the .close-button class has no focus indicator:

I also noticed that the invisible buttons also have no focus indicator.

To Reproduce
Steps to reproduce the behavior:

1. Go to primer.style/css
2. Force focus on either the invisible button or the close button
3. See that there is no focus indicator even when navigating by keyboard + check the styles and see that both the outline and the box shadow are removed when focused

Expected behavior
All buttons should comply with WAI-ARIA 6.2 Discernible and Predictable Keyboard Focus

Screenshots

[emplums]

Note: In PRC I went ahead and used the default box-shadow-focus for the focus indicator, if we end up deciding to use something else please ping me in this issue so I can update it in React land :)

[emplums]

Going to go ahead and add the default focus outline to the ButtonInvisible in PRC as well - this still needs to be updated in Figma and Primer CSS cc @ashygee and @simurai

[ashygee]

@emplums this has been updated today in Figma

I'm cheating a bit in Figma because the shadow won't show without a fill color so the bg-canvas fill has been applied to this to change with color modes.