feat: add CSS/SASS variables for ul > li vertical alignment (#54)

Author: ghiscoding

lib/src/styles/_variables.scss

@@ -31,11 +31,13 @@ $ms-drop-input-margin-top: 0.3rem !default;
 $ms-drop-optgroup-font-weight: bold !default;
 $ms-drop-list-margin: 0px !default;
 $ms-drop-list-padding: 0px !default;
-$ms-drop-list-item-level1-padding-left: 28px !default;
 $ms-drop-option-divider-padding: 0 !default;
 $ms-drop-option-divider-border-top: 1px solid #e9ecef !default;
 $ms-drop-margin-top: -1px !default;
 $ms-drop-padding: 0 !default;
+$ms-drop-list-item-align-items: center !default;
+$ms-drop-list-item-display: list-item !default;
+$ms-drop-list-item-level1-padding-left: 28px !default;
 $ms-drop-list-item-padding: 0.25rem 8px !default;
 $ms-drop-list-item-disabled-filter: Alpha(Opacity = 35) !default;
 $ms-drop-list-item-disabled-opacity: 0.35 !default;

lib/src/styles/multiple-select.scss

@@ -213,9 +213,10 @@
 
     > li {
       background-image: none;
-      display: list-item;
       list-style: none;
       position: static;
+      align-items: var(--ms-drop-list-item-align-items, $ms-drop-list-item-align-items);
+      display: var(--ms-drop-list-item-display, $ms-drop-list-item-display);
       padding: var(--ms-drop-list-item-padding, $ms-drop-list-item-padding);
 
       .disabled {