Break mixins.less up into smaller modules.

Move all deprecated vendor prefix mixins to less/mixins/vendor-prefixes.less
Create mixins directory and move partials to that directory.

FIXES #12994

less/mixins/list-group.less

@@ -0,0 +1,27 @@
+// List Groups
+
+.list-group-item-variant(@state; @background; @color) {
+  .list-group-item-@{state} {
+    color: @color;
+    background-color: @background;
+
+    a& {
+      color: @color;
+
+      .list-group-item-heading { color: inherit; }
+
+      &:hover,
+      &:focus {
+        color: @color;
+        background-color: darken(@background, 5%);
+      }
+      &.active,
+      &.active:hover,
+      &.active:focus {
+        color: #fff;
+        background-color: @color;
+        border-color: @color;
+      }
+    }
+  }
+}