Feature request #26681

* reduce # of spacing rules by combining x & y with t r b l

* Remove unnecessary spaces

- Renames .pos-f-t to .fixed-top, adds .fixed-bottom and .sticky-top
- Updates utilities Sass to be split across more files (one for position, sizing, and spacing)

Adding 25%, 50%, and 75% to the mix for later use with cards

Adding 25%, 50%, and 75% to the mix for later use with cards

* Remove comment that duplicated some code
* Use transition mixin whenever possible
* Create a new function to reduce duplication
* Use the new `breakpoint-infix` method

* Clean up some utilities

- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies

* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication

* Fix outdated classes to use newer ones

* make spacer utils responsive by grid tier
* update scale to add two levels, document them
* change responsive spacing utils to avoid the xs abbreviation in the class name for that tier
* update code snippet to match source
* update usage in our docs
* linter
* docs updates

Rename margin and padding utilities from .{property}-{side} to .{property}{side} without the extra dash

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