Added sizing variables

Author: extrabright

scss/core/variables/_sizing.scss

@@ -29,13 +29,20 @@ $percentage-sizers: map-merge((
     15\/10: 150%,
 ), $percentage-sizers);
 
-// This variable affects the `.pc-*`, `.max-pc-*` classes.
+// This variable affects the `.w-screen-*`, `.max-w-screen-*` classes.
+$screen-width-sm: 640px !default;
+$screen-width-md: 768px !default;
+$screen-width-lg: 1024px !default;
+$screen-width-xl: 1280px !default;
+$screen-width-xxl: 1536px !default;
+
+// Map
 $screen-widths: () !default;
 // stylelint-disable-next-line scss/dollar-variable-default
 $screen-widths: map-merge((
-    screen-sm: 640px,
-    screen-md: 768px,
-    screen-lg: 1024px,
-    screen-xl: 1280px,
-    screen-xxl: 1536px
+  screen-sm: $screen-width-sm,
+  screen-md: $screen-width-md,
+  screen-lg: $screen-width-lg,
+  screen-xl: $screen-width-xl,
+  screen-xxl: $screen-width-xxl
 ), $screen-widths);