* 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.

* Making use of `prefers-reduced-motion` media query

As discussed in #25249 - if a user (Who is using Safari / iOS) requests
reduced motion in their system settings, we should avoid transitions.

* Ignoring prefers reduced motion for CSS Linting
* Updating copy clarifying the reduce motion functionality in accessibility.md

* add missing properties

list of new properties:
align-content
animation-fill-mode
appearance
column-count
column-fill
column-gap
column-rule
column-rule-color
column-rule-style
column-rule-width
column-span
column-width
columns
direction
fill
flex-shrink
orphans
page-break-after
page-break-before
page-break-inside
perspective
src
stroke
unicode-bidi
widows

* fix invalid JSON config

add curly brackets

* Update .stylelintrc

This checks if all variables are `!default`.

Also, refactor ESLint and stylelint config files; now everything will work from our root dir.

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