feat(components): [input] add prefix, suffix, prepend and append props for simple plain text.

[drylint]

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At present, <el-input> supports prefix, suffix, prepend and append slots, it's useful to insert something into <el-input> components.

But in many times, we might just want to insert a plain text, like <el-dialog> accept title prop and header (called title before) slot, we can use title prop for plain text, and use header slot to render complex content.

So, added these props the same name with the slot, and the slot has a higher priority.

<!-- use slots -->
<el-input
  v-model="inputValue"
  placeholder="Please input"
>
  <template #prepend>Http://</template>
  <template #append>.com</template>
</el-input>

<!-- use props -->
<el-input
  v-model="inputValue"
  placeholder="Please input"
  prepend="Http://"
  append=".com"
/>

In addition, use an Array of object to render a form list via v-for directive, it's convenient to pass prefix, suffix, prepend and append as a property of a object to render plain text.

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