Improve and extend focus styles for Windows High Contrast mode

[afercia]

See [44544] and #41286.

[44544] introduced new focus styles for the Windows High Contrast mode only for the main user interface elements like buttons, links, media views elements.

In order to cover all the controls in the admin, there's the need to explore the admin parts that make use of uncommon styles or don't inherit the relevant CSS, for example the Customizer, the themes browser, etc.

Some thorough testing on Windows would be greatly appreciated.

[afercia]

Discussed during today's accessibility bug scrub and agreed to move this ticket to Future Release as it's something to address. Will need research and testing.

[afercia]

See comments 11 and 12 from #47117.

[afercia]

In 48293:

Accessibility: Improve the focus style for Windows High Contrast mode in various parts of the admin interface.

Continues the introduction in core of new focus styles dedicated to Windows High Contrast mode. The new styles use a transparent CSS outline.
This change covers some parts of the interface for the meta boxes, Widgets, and the Customizer.

Props joedolson, kjellr, antpb, mikeschroder, Hareesh Pillai.
See #41286, #45910.
Fixes #47117.

[joedolson]

Work has been done on this, it needs assessment to identify whether more changes are needed.