change deprecated slot-scope to v-slot

Directive `slot-scope` is deprecated as of Vue v2.6.0. See [here](https://vuejs.org/v2/guide/components-slots.html) for details. Currently tiptap depends on Vue ^2.6.10. So this commit introduces new directive `v-slot` on README.md and examples.

examples/Components/Routes/FloatingMenu/index.vue

@@ -1,8 +1,7 @@
 <template>
   <div class="editor">
-    <editor-floating-menu :editor="editor">
+    <editor-floating-menu :editor="editor" v-slot="{ commands, isActive, menu }">
       <div
-        slot-scope="{ commands, isActive, menu }"
         class="editor__floating-menu"
         :class="{ 'is-active': menu.isActive }"
         :style="`top: ${menu.top}px`"