Icon: Adding font-display to @font-face declaration (#16805)

packages/theme-chalk/src/common/var.scss

@@ -104,6 +104,7 @@ $--fill-base: $--color-white !default;
 /* Typography
 -------------------------- */
 $--font-path: 'fonts' !default;
+$--font-display: 'auto' !default;
 /// fontSize|1|Font Size|0
 $--font-size-extra-large: 20px !default;
 /// fontSize|1|Font Size|0

packages/theme-chalk/src/icon.scss

@@ -5,7 +5,8 @@
   src: url('#{$--font-path}/element-icons.woff') format('woff'), /* chrome, firefox */
        url('#{$--font-path}/element-icons.ttf') format('truetype'); /* chrome, firefox, opera, Safari, Android, iOS 4.2+*/
   font-weight: normal;
-  font-style: normal
+  font-display: $--font-display;
+  font-style: normal;
 }
 
 [class^="el-icon-"], [class*=" el-icon-"] {