Add debug warnings for <p> end tag omission parsing rules

[marvinhagemeister]

The HTML spec has some special cases surrounding the <p> tag: https://html.spec.whatwg.org/multipage/grouping-content.html#the-p-element

A p element’s end tag can be omitted if the p element is immediately followed by an address, article, aside, blockquote, details, div, dl, fieldset, figcaption, figure, footer, form, h1, h2, h3, h4, h5, h6, header, hgroup, hr, main, menu, nav, ol, p, pre, search, section, table, or ul element, or if there is no more content in the parent element and the parent element is an HTML element that is not an a, audio, del, ins, map, noscript, or video element, or an autonomous custom element.

This leads to cases where:

 <p>
   <h1>hello</h1>
 </p>

...will be turned into this by the browser:

<p></p>
<h1>Hello</h1>
<p></p>

This confuses Preact as it's sort of a hydration mismatch and the vnode tree doesn't match the HTML anymore.

[nmain]

Could this include other non-<p> scenarios? For instance, this:

<table><tbody><tr><table></table></tr></tbody></table>

can be created in (p)react clientside or raw dom. But if rendered to string on a server, and then later parsed by the browser's HTML parser, it will parse as:

<table><tbody><tr></tr></tbody></table><table></table>

[marvinhagemeister]

Sure, that's a good idea!