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Fixes #18609

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- Renames old and unused $nav-pills-active-link-hover-* for
$nav-pills-active-link-* ones
- Puts those vars to use in place of defaults $component-* vars on nav
pills

- Add new $nav-tabs-border-width for the bottom border on the .nav-tabs
parent class
- Use that new variable for the negative margin on nav-items within it
for consistent customization
- Drop the $nav-tabs-link-border-width for the new variable so it’s all
tied together

So folks can customize the distance between nav links in inline navs.

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- Less nesting with more specific classes
- New class names for most elements to avoid unnecessary and potentially confusing shorthand
- Require new class and markup for the description (though it's only necessary for disable states, it's now part of the entire component's markup for all states just in case)
- Change up colors and variables for regular and disabled states

Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.

Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x

Introduce new variable: $breadcrumb-item-padding

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$navbar-padding-horizontal => $navbar-padding-x
$navbar-padding-vertical   => $navbar-padding-y

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