fix: change z-index to style variable (#412)

Author: mengxiong10

src/style/index.scss

@@ -105,7 +105,7 @@
   margin-top: 1px;
   margin-bottom: 1px;
   box-shadow: 0 6px 12px rgba(0, 0, 0, 0.175);
-  z-index: 2001;
+  z-index: $popup-z-index;
 }
 
 .#{$namespace}-datepicker-sidebar {

src/style/var.scss

@@ -5,6 +5,8 @@ $primary-color: #1284e7 !default;
 
 $today-color: mix(#fff, $primary-color, 10%) !default;
 
+$popup-z-index: 2001 !default;
+
 $input-border-color: #ccc !default;
 $input-color: #555 !default;
 $input-hover-border-color: #409aff !default;