Ensure that start/end tag handler does not include tags in the previous
paragraph.

Provide special handling for tags like hr that never have content.

Use sets for block tag lists as they are much faster when comparing
if an item is in the list.

Fixes #1053.

Fixes #1055.

This action checks that an update was made to any file in docs/change_log/
but only if changes were made to files in markdown/. Presumably,
any changes outside of markdown/ do not affect the behavior and do not
require a notation in the change_log.

If the proper permissions are available, a comment is added to the PR informing
the PR author that an update to the changelog is missing. However, any PR's from
forks do not have permission on the pull_request event. Unfortunately, the
pull_request_target event doesn't seem to work at all. However, as the action
only attempts to add a comment on failure, we only get a failure in the correct
conditions, even if the error message is related to auth.

Fixes #1049

Use the list of tags defined in the core by the md_in_html extension.
This ensures that the lists do not diverge and allows users and/or
extensions to expand the list in the core and have that change affect
the extension. Fixes #1047.

Fixes #1040 and fixes #1045.

By calling str on all stash elements we ensure they don't raise an error.
Worse case, soemthing like `<Element 'div' at 0x000001B2DAE94900>` gets
inserted into the output. However, with the override in the md_in_html
extension, we actually serialize and reinsert the original HTML. Worse case,
an HTML block which should be parsed as Markdown gets skipped by the
extension (`<div markdown="block"></div>` gets inserting into the output).

The tricky part is testing as there should be no known cases where this
ever occurs. Therefore, we forefully pass an etree Element directly to
the method in the test. That said, as #1040 is unresolved at this point,
I have tested locally with a real existing case and it works well.

Related to #1040.