Move box-shadow and transition mixins out of the main mixins file and into their own files

scss/mixins/_box-shadow.scss

@@ -0,0 +1,5 @@
+@mixin box-shadow($shadow...) {
+  @if $enable-shadows {
+    box-shadow: $shadow;
+  }
+}

scss/mixins/_transition.scss

@@ -0,0 +1,9 @@
+@mixin transition($transition...) {
+  @if $enable-transitions {
+    @if length($transition) == 0 {
+      transition: $transition-base;
+    } @else {
+      transition: $transition;
+    }
+  }
+}