Caption filter should use existing <figcaption> and keep comments

Problem/Motivation

1) The caption filter currently always wraps the captioned element in a <figure> block to which it adds its own <figcaption>. This creates unnessecarily nested markup in case the element itself is already presented as/contains a <figure> with a <figcaption>.

2) Additionally, the $node->C14N() call strips all comments, which includes Twig debug comments (we can deal with that) and compatibility comments like those used for responsive images: <!--[if IE 9]><video style="display: none;"><![endif]--> (this is bad and will cause problems).

Example setup for 1) using entity_embed:

1. Media type "Image" with a template like <figure>{{ content.field_image }}<figcaption>{{ name }}</figcaption></figure>
2. Some content in which the media entity is embedded with a specified custom caption

Expected result:
<figure>{{ content.field_image }}<figcaption>{{ my overridden caption}}</figcaption></figure>

Actual result (using default filter-caption.html.twig):

<figure>
  <figure>
    {{ content.field_image }}
    <figcaption>{{ name }}</figcaption>
  </figure>
  <figcaption>{{ my overridden caption }}</figcaption>
</figure>

Proposed resolution

1) Re-use an already present <figcaption> element.
2) Keep comments when building $altered_html.

Remaining tasks

Discuss what we need/want to do. Review.