fix(d-form-textarea): add the 'value' prop

[BenoitZugmeyer]

Passing a value prop to the FormTextarea didn't do anything for two reasons:

• the value prop wasn't declared, so it was always undefined

• the localValue prop wasn't passed to the underlying textarea component, so the textarea wasn't controlled (it did not show the current value).

Using a v-model on the textarea simplifies the code a bit by sending and updating localValue at the same time.

PS - I read the contributing guidelines after making a fix for this issue, that's why I didn't take time to discuss about it first. Of course, we can still discuss this, and feel free to discard the PR completely.