* Remove unnecessary float

- since parent btn-group is displayed as inline-flex, the child elements are placed left to right by default.

* Cleanup of reset rounded corners

- Remove `border-radius :0` since it is enough by each reset of left and right side
- A `.dropdown-toggle` selector should not be used for reset rounded corner since it does not have border-radius property. Instead use .dropdown-toggle as negation pseudo-class.

Referencing #23728, styling grouped radio and checkbox buttons should not require the data-toggle attribute.

1. move a couple of them before the more specific ones.
2. change nesting to be under the same parent selector
3. use the ampersand more

The option is set to `always-single-line` in stylelint-config-standard which we extend.

* add margin-bottom to .btn of a vertical button group

* match margins of radio/checkbox buttons in a btn-group to the actual buttons

* add comment

Reduce z-indexes in button-group, input-group, list-group, and pagination to the minimum necessary (#24315)

These were using `z-index: 2` to "Place active items above their
siblings for proper border styling". However, using `z-index: 1` is
sufficient for accomplishing that goal.

In input-group, there were also three `z-index: 3` rules for the
hover/focus/active states. I reduced these to `z-index: 2` since they
just needed to be "one more than normal" (i.e. one more than what is now
`z-index: 1` after my changes).

These changes can be verified by viewing the documentation pages for
Button group, Input group, List group, and Pagination before and after
this commit and observing that the active elements are still "above"
their siblings, so their borders look correct.

* use a mixin there
* revamp .btn-link to match default and outline variants
* remove unnecessary properties

fron -> front

* no more outline overrides
* rename old .open to .show

Fixes #21699

shoutout to @Starsam80 for the fix

- Fixes #20298 for vertical button group spacing
- Fixes #20784 for horizontal (default) button group spacing
- Alternate fix to #20823 which only applied a fix for vertical button groups

- Really we didn't need to do any of that strip units stuff to generate six new variables, two for each button size
- Using sibling selectors, we can target those split button dropdown toggles using the .dropdown-toggle-split class, and adjust padding and margin as needed
- Now, we nuke the margin-left from the ::after generated caret and tighten up the padding so that those split toggles don't look huge next to their main button

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- New vars
- New function for stripping units so we can combine rems and ems in math functions
- Add new classes for sizing and spacing around the split dropdown toggle so that the caret isn't misaligned

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Both widgets need to use the same border width because of input groups.
Thus:
$btn-border-width   => $input-btn-border-width
$input-border-width => $input-btn-border-width

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Fixes half of #18425
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- no real need for everything to be 100% shared
- padding looked and felt too large for inputs but not for buttons
- tying forms and buttons seems fine, but throwing in pagination feels wrong