updating to 4.0

New design with breaking changes, gulp 4, all dependencies updated, dev environment changed, and many more changes

vendor/bootstrap/scss/mixins/_size.scss

@@ -0,0 +1,6 @@
+// Sizing shortcuts
+
+@mixin size($width, $height: $width) {
+  width: $width;
+  height: $height;
+}