* add 'lighter' and 'bolder' font weight classes
these are 2 special values for font weight, that will give their content a font-weight value of 100 more or less than their inherited font-weight.
probably doesn't fully fulfill this issue, https://github.com/twbs/bootstrap/issues/23969, but it's a start
* Update .stylelintrc
* add 'lighter' and 'bolder' variables per @mdo 's request.
* 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* Fix outdated classes to use newer ones
- Rather than mix multiple properties in our color utilities, this splits all color and all background utils into separate classes.
- Adds new .text-white class to help lighten text color for darker backgrounds
