Cleanup/fix after the sr-only to visually-hidden renaming (twbs#31359)

* sr-only -> visually-hidden cleanup

It seems the old screen-readers.md file was left behind, and forgot to rename the mixin

* Fix broken mixins for visually-hidden

scss/_mixins.scss

@@ -12,7 +12,7 @@
 @import "mixins/breakpoints";
 @import "mixins/image";
 @import "mixins/resize";
-@import "mixins/screen-reader";
+@import "mixins/visually-hidden";
 @import "mixins/reset-text";
 @import "mixins/text-truncate";
 

scss/mixins/_screen-reader.scss → scss/mixins/_visually-hidden.scss

@@ -1,11 +1,11 @@
 // stylelint-disable declaration-no-important
 
-// Only display content to screen readers
+// Hide content visually while keeping it accessible to assistive technologies
 //
 // See: https://a11yproject.com/posts/how-to-hide-content/
 // See: https://hugogiraudel.com/2016/10/13/css-hide-and-seek/
 
-@mixin visually-hidden {
+@mixin visually-hidden() {
   position: absolute !important;
   width: 1px !important;
   height: 1px !important;
@@ -21,7 +21,7 @@
 //
 // Useful for "Skip to main content" links; see https://www.w3.org/TR/2013/NOTE-WCAG20-TECHS-20130905/G1
 
-@mixin visually-hidden-focusable {
+@mixin visually-hidden-focusable() {
   &:not(:focus) {
     @include visually-hidden();
   }