Implements some HTMLTextAreaElement attributes

These attributes all reflect their own related content values, with the
exception of defaultValue, which acts as an alias for its IDL
textContent attribute.

Many of these do have default values and constraints which are currently unimplemented.
Implements multi line text input for TextArea
Updates WPT to expect newly passing tests

Addresses reviews

More review addressing
HTMLTextArea: Implements a dirty value flag
auto merge of #4152 : mttr/servo/textview, r=jdm

Fixes #3918 

Can be tested in `tests/html/textarea.html`. Also implemented some content reflecting IDL attributes for HTMLTextAreaElement while I was in there.

There are some major problems with TextInput when Multiple is enabled that I haven't addressed here, but I'm prepared to open up a follow-up issue.