The previous logic was problematic for amsmath blocks nested in other blocs (such as blockquotes)

The new parsing code now principally follows the logic in `markdown_it/rules_block/fence.py`
(see also https://spec.commonmark.org/0.30/#fenced-code-blocks),
except that:

1. it allows for a closing tag on the same line as the opening tag, and
2. it does not allow for an opening tag without closing tag (i.e. no auto-closing)

The `allowed` option accepts a list of allowed attribute names.
If not ``None``, any attributes not in this list will be removed
and placed in the token's meta under the key `"insecure_attrs"`.

Co-authored-by: maximkurbatov <[email protected]>
Co-authored-by: Chris Sewell <[email protected]>