[Textarea] Add a prop to make borderless and padding-less

[NozomuIkuta]

We came to have a usecase where textarea and its submit button are both inside a bordered box.
So, we have to hide textarea's own border and remove all padding around it.

[kiaking]

Maybe... :variant prop might be good name...?

interface Props {
  variant: 'default' | 'clear'
}

[NozomuIkuta]

@kiaking

Can we close this issue or should we implement this feature regardless of changes on our roadmap?

[kiaking]

Let's keep this. It is still useful in various places 👀