Several small tweaks to the props handling:
* Use destructuring instead of `delete`
* Emphasize the `element` as a primary prop
* Document `textarea` as supported
Fixes https://github.com/vector-im/riot-web/issues/8250
This keeps all fields in line with the design without them having to defining it twice. The option is kept in the first place as some fields might want to override the placeholder to be longer than the label or something.
The label moves into the border on focus and after being filled. A valid color
is applied to the label and input border. Other states like invalid can be added
later as needed.
Adapted from @ara4n's experiment into a React component with a CSS only
approach.
