refactor: add @modal-footer-padding-{vertical,horizontal}

components/modal/style/modal.less

@@ -1,5 +1,7 @@
 @dialog-prefix-cls: ~'@{ant-prefix}-modal';
 @table-prefix-cls: ~'@{ant-prefix}-table';
+@modal-footer-padding-vertical: 10px;
+@modal-footer-padding-horizontal: 16px;
 
 .@{dialog-prefix-cls} {
   .reset-component;
@@ -89,7 +91,7 @@
   }
 
   &-footer {
-    padding: 10px 16px;
+    padding: @modal-footer-padding-vertical @modal-footer-padding-horizontal;
     text-align: right;
     border-top: @border-width-base @border-style-base @border-color-split;
     border-radius: 0 0 @border-radius-base @border-radius-base;