sass: fix slash division deprecation in font-awesome

Replace deprecated `/` division with `calc()` to fix Dart Sass 2.0
deprecation warnings. See https://sass-lang.com/d/slash-div

Author: stickies-v

_sass/vendor/font-awesome/_fixed-width.scss

@@ -1,6 +1,6 @@
 // Fixed Width Icons
 // -------------------------
 .#{$fa-css-prefix}-fw {
-  width: (18em / 14);
+  width: calc(18em / 14);
   text-align: center;
 }

_sass/vendor/font-awesome/_larger.scss

@@ -3,8 +3,8 @@
 
 /* makes the font 33% larger relative to the icon container */
 .#{$fa-css-prefix}-lg {
-  font-size: (4em / 3);
-  line-height: (3em / 4);
+  font-size: calc(4em / 3);
+  line-height: calc(3em / 4);
   vertical-align: -15%;
 }
 .#{$fa-css-prefix}-2x { font-size: 2em; }

_sass/vendor/font-awesome/_list.scss

@@ -11,9 +11,9 @@
   position: absolute;
   left: -$fa-li-width;
   width: $fa-li-width;
-  top: (2em / 14);
+  top: calc(2em / 14);
   text-align: center;
   &.#{$fa-css-prefix}-lg {
-    left: -$fa-li-width + (4em / 14);
+    left: calc(-1 * $fa-li-width + calc(4em / 14));
   }
 }

_sass/vendor/font-awesome/_variables.scss

@@ -9,7 +9,7 @@ $fa-css-prefix:       fa !default;
 $fa-version:          "4.5.0" !default;
 $fa-border-color:     #eee !default;
 $fa-inverse:          #fff !default;
-$fa-li-width:         (30em / 14) !default;
+$fa-li-width:         calc(30em / 14) !default;
 
 $fa-var-500px: "\f26e";
 $fa-var-adjust: "\f042";