WIP: HTML API: Add function to balance tags ala force_balance_tags()

[dmsnell]

• adds closing tags to open elements when they implicitly close
• does not add closing tag when hitting unsupported content that would require reconstructing active formatting elements

<p><div><button>First<button><b here>Second
↧
<p></p><div><button>First</button><button><b here>Second</b></button></div>

<p><div>asdf</p><p>Inside</p></figcaption><button>First<button><b here>Second</span></button>
↧
<p></p><div>asdf<p>Inside</p><button>First</button><button><b here>Second</button></div>