Add "renderIcon" attribute to NButton.

[kvii]

This function solves the problem (这个功能解决的问题)

The following code is a simple demo to create a IconButton.
As you see, even the simplest demo will take 7 lines. It's to boring.

<NButton>
  <template #icon>
    <NIcon>
      <SomeIcon />
    </NIcon>
  </template>
</NButton>

Expected API (期望的 API)

Add a "renderIcon" attribute to NButton.
Then we can write come code like below:

<NButton
  :renderIcon="renderIcon(SomeIcon)"
/>

That makes code clearer and easier to understand.