Add Attributes formAction, formEncType, formMethod and formTarget

This resolves #2425.

Test plan: This was added to a webpage and it worked:
```html
<form>
  <input name='text'/>
  <button
    formAction='anotherUrl.html'
    formEncType='multipart/form-data'
    formMethod='POST'
    formTarget='_blank'
  >
    Submit
  </button>
</form>
```
This is the supported elements section, attributes are farther down :)