[Feature Request] `el-input-number` support slots: prepend & append

### Existing Component
是

### Component Name
el-input-number

### Description
# Expected:
`el-input-number` can custom by prepend & append slot.
# Example:
```

Last
days

```
by default, it is still the `+` & `-`。
# other
I found some issues shows that this function is supported before, I hope developers & maintaners can give a reason but not only tell me that it was not supported anymore.

*related issues before:*
- https://github.com/ElemeFE/element/issues/11792#issuecomment-401223387
- https://github.com/ElemeFE/element/issues/2652