Focus ring helper and utilities

• Adds new global Sass variables for :focus styling
• Reassigns $input-btn-focus-* variables to new global variables (come v6, we'll simplify these variables)
• Adds focus styles to all navigation styles (including base .nav, .nav-pills, and .nav-tabs)

Closes #32357.

Live previews

• https://deploy-preview-33125--twbs-bootstrap.netlify.app/docs/5.2/helpers/focus-ring/
• https://deploy-preview-33125--twbs-bootstrap.netlify.app/docs/5.2/components/navs-tabs/