Add DomHtmlTextAreaElement bindings

I filed https://bugzilla.xamarin.com/show_bug.cgi?id=7754 and https://bugzilla.xamarin.com/show_bug.cgi?id=11693 because I was unable to access some properties on input and textarea elements from the DOM. The input element properties were fixed, but textarea elements were left out. I added the appropriate bindings as I needed those properties ASAP.