fix(helpers): only deep merge plain objects

[cgodo]

Description

The defaults composable uses the mergeDeep helper to merge defaults. However, it is not able to merge properties with HTML elements, like for example, the attach property of all overlays, because it throws a maximum call stack size exceeded.

Changed the mergeDeep utility to handle objects with circular references and to avoid traversing Nodes. Also added two new tests to check these changes.

fixes #20278

Markup:

<script setup>
  const defaultsA = { VMenu: { attach: document.body } }
  const defaultsB = { VMenu: { attach: document.getElementById('app') } }
</script>

<template>
  <v-menu location="right">
    <template #activator="{ props }">
      <v-btn v-bind="props" class="ma-5">Menu</v-btn>
    </template>
    <v-list>
      <v-list-item title="Menu item" />
      <v-list-item title="Menu item" />
    </v-list>
  </v-menu>

  <v-defaults-provider :defaults="defaultsA">
    <v-defaults-provider :defaults="defaultsB">
      <v-menu location="right">
        <template #activator="{ props }">
          <v-btn v-bind="props" class="ma-5">Menu with defaults provider</v-btn>
        </template>
        <v-list>
          <v-list-item title="Menu item" />
          <v-list-item title="Menu item" />
        </v-list>
      </v-menu>
    </v-defaults-provider>
  </v-defaults-provider>
</template>

[KaelWD]

This function is called quite a lot and circular reference checking seems like it could add a fair bit of overhead, I think just a Object.getPrototypeOf(val) === Object.prototype might be good enough for detecting non-plain objects.

[cgodo]

Yes, your suggestion would be simpler and even work for more non-plain objects, not just Nodes, so circular checks won't be necessary. I made the changes in mergeDeep and updated tests.