Fixed SASS error

This fixes the SASS error caused by the `&[dir="rtl"]` selector in
the `_single.scss` and `_multiple.scss` files. We were expecting
these files to be nested within the `.select-container` class, which
would have made this a non-error as the `&` would refer to the
`.select2-container` class that was wrapping the `@import`.

This moves the `@import` statements into the `.select2-container`
class, so this no longer triggers an error in the Ruby version of
SASS.

This closes https://github.com/select2/select2/issues/2896

dist/css/select2.css

@@ -4,41 +4,37 @@
   margin: 0;
   position: relative;
   vertical-align: middle; }
-
-.select2-selection--single {
+  .select2-container .select2-selection--single {
     box-sizing: border-box;
     cursor: pointer;
     display: block;
     height: 28px;
     user-select: none;
     -webkit-user-select: none; }
-  .select2-selection--single .select2-selection__rendered {
+    .select2-container .select2-selection--single .select2-selection__rendered {
       display: block;
       overflow: hidden;
       padding-left: 8px;
       padding-right: 20px;
       text-overflow: ellipsis; }
-
-[dir="rtl"] .select2-selection--single .select2-selection__rendered {
+  .select2-container[dir="rtl"] .select2-selection--single .select2-selection__rendered {
     padding-right: 8px;
     padding-left: 20px; }
-
-.select2-selection--multiple {
+  .select2-container .select2-selection--multiple {
     box-sizing: border-box;
     cursor: pointer;
     display: block;
     min-height: 32px;
     user-select: none;
     -webkit-user-select: none; }
-  .select2-selection--multiple .select2-selection__rendered {
+    .select2-container .select2-selection--multiple .select2-selection__rendered {
       display: inline-block;
       overflow: hidden;
       padding-left: 8px;
       text-overflow: ellipsis; }
-
-.select2-search--inline {
+  .select2-container .select2-search--inline {
     float: left; }
-  .select2-search--inline .select2-search__field {
+    .select2-container .select2-search--inline .select2-search__field {
       border: none;
       font-size: 100%;
       margin-top: 5px; }

src/scss/core.scss

@@ -5,10 +5,10 @@
   margin: 0;
   position: relative;
   vertical-align: middle;
-}
 
   @import "single";
   @import "multiple";
+}
 
 @import "dropdown";