Implements basic form resetting

We can reset `<input type=text>` fields! I wish I could've done something with checkboxes, but unfortunately, that's it for now.

In addition to that, this PR implements `HTMLInputAttribute.defaultValue`, updates wpt-test to expect passing tests as a result of that implementation, and fixes an index error crash with text inputs.

edit: also includes an html example where one may lazily watch form resets in action: ` tests/html/form_reset_handsfree.html`