💄 Add less variables for button shadow

close #14172

components/button/style/mixin.less

@@ -25,8 +25,8 @@
 
 .button-variant-primary(@color; @background) {
   .button-color(@color; @background; @background);
-  text-shadow: 0 -1px 0 rgba(0, 0, 0, 0.12);
+  text-shadow: @btn-text-shadow;
-  box-shadow: 0 2px 0 rgba(0, 0, 0, 0.045);
+  box-shadow: @btn-primary-shadow;
 
   &:hover,
   &:focus {
@@ -162,7 +162,7 @@
   user-select: none;
   transition: all 0.3s @ease-in-out;
   position: relative;
-  box-shadow: 0 2px 0 rgba(0, 0, 0, 0.015);
+  box-shadow: @btn-shadow;
   > .@{iconfont-css-prefix} {
     line-height: 1;
   }

components/style/themes/default.less

@@ -132,6 +132,9 @@
 @btn-font-weight : 400;
 @btn-border-radius-base : @border-radius-base;
 @btn-border-radius-sm : @border-radius-base;
+@btn-shadow: 0 2px 0 rgba(0, 0, 0, .015);
+@btn-primary-shadow: 0 2px 0 rgba(0, 0, 0, 0.045);
+@btn-text-shadow: 0 -1px 0 rgba(0, 0, 0, 0.12);
 
 @btn-primary-color : #fff;
 @btn-primary-bg : @primary-color;