Filter a tree

[kellerza]

Would you be interested to add filtering to a tree?

Example of filter behavior here

This PR moves flattenedItem and flattenItems to utils and adds flattenFilter to achieve this

flattenFilter needs a filter function and a list of 'visible' branches.
Output the flattenedItems & expanded keys (maybe this could be useful in the future, but no need to do anything with this)
Once filtered, it would be great to expand a certain branch (adding the key to forceVisible achieves this)

I would appreciate some guidance on the API

[kellerza]

I've taken a new approach. Instead of modifying TreeRoot, I madea new component responsible for filtering the input to TreeRoot, it's called TreeData, you pass it items and some filters props and it returns filteredItems in the slot and modifies expanded

  <TreeData
    ref="treedata"
    v-slot="{ filteredItems }"
    v-model:expanded="expanded"
    :items="allItems"
    :filter="textFilter"
    :get-text="(i) => i.title"
  >
    <TreeRoot
      v-model:expanded="expanded"
      :items="filteredItems"
    >
   ...
    </TreeRoot>
  </TreeData>

Might be a cleaner approach?