fix(sass): allow overriding $spacers

fixes #10303

packages/vuetify/src/styles/settings/_variables.scss

@@ -9,15 +9,19 @@ $border-radius-root: 4px !default;
 
 $spacer: 4px !default;
 
-$spacers: ();
-@for $i from 0 through 12 {
-  $spacers: map-merge($spacers, ($i: $spacer * $i))
-};
-
-$negative-spacers: ();
-@for $i from 1 through 12 {
-  $negative-spacers: map-merge($negative-spacers, ("n" + $i: -$spacer * $i))
-};
+$spacers: () !default;
+@if (type-of($spacers) == list) {
+  @for $i from 0 through 12 {
+    $spacers: map-merge($spacers, ($i: $spacer * $i))
+  }
+}
+
+$negative-spacers: () !default;
+@if (type-of($negative-spacers) == list) {
+  @for $i from 1 through 12 {
+    $negative-spacers: map-merge($negative-spacers, ("n" + $i: -$spacer * $i))
+  }
+}
 
 $grid-breakpoints: () !default;
 $grid-breakpoints: map-deep-merge(