Better handling of textareas

[Rich-Harris]

This primarily fixes #582, but also address some other behaviour around textareas:

value and children are treated equivalently

<!-- these are the same, as far as Svelte is concerned -->
<textarea value='{{foo}}'/>
<textarea>{{foo}}</textarea>

A textarea with both a value attribute and children will fail validation.

textareas cannot have child elements or if/each blocks

As a corollary to the above:

<!-- these are the same -->
<textarea>
  <p>surprise! this isn't an element</p>
</textarea>

<textarea value="\n\t<p>surprise! this isn't an element</p>\n"/>

SSR

Before (incorrect):

<textarea value="42"></textarea>

After:

<textarea>42</textarea>