Add horizontal list group modifier (#27970)

Alternate take to #27683 that builds the modifier class based on min-width, such that list groups are stacked until the class's breakpoint is reached. This is less verbose, matches our primary responsive approach, and prevents an extra class. Unfortunately, I believe supporting flush list groups is too much code here, so I've skipped that and made a note in the docs.

Also added examples generate from our breakpoints data file with a protip for how to do equal width items.

scss/mixins/_border-radius.scss

@@ -37,3 +37,15 @@
     border-bottom-left-radius: $radius;
   }
 }
+
+@mixin border-top-right-radius($radius) {
+  @if $enable-rounded {
+    border-top-right-radius: $radius;
+  }
+}
+
+@mixin border-bottom-left-radius($radius) {
+  @if $enable-rounded {
+    border-bottom-left-radius: $radius;
+  }
+}