feat(components): add pattern-container slot to Background

# What's changed?

* `pattern-container` allows for complete overwrite of the pattern of Background

packages/vue-flow/src/additional-components/Background/Background.vue

@@ -60,6 +60,7 @@ export default {
       width: `${width > 100 ? 100 : width}%`,
     }"
   >
+    <slot :id="patternId" name="pattern-container">
       <pattern
         :id="patternId"
         :x="background.xOffset"
@@ -80,7 +81,8 @@ export default {
           </svg>
         </slot>
       </pattern>
+    </slot>
     <rect :x="x" :y="y" width="100%" height="100%" :fill="`url(#${patternId})`" />
-    <slot></slot>
+    <slot :id="patternId" />
   </svg>
 </template>