fix: treat custom element names as block-level HTML

[BetterAndBetterII]

Summary

Custom element names are required to contain a hyphen. is_block_level() and md_in_html previously only recognized a fixed list of block tags, so a standalone <a-b>…</a-b> was wrapped in <p> and the markdown attribute on those tags was ignored.

Treat any tag name that contains a hyphen as block-level (same treatment as <div>). Inner Markdown still requires the markdown / markdown="1" attribute. Inline use mid-paragraph stays span-level.

Test plan

• python -m unittest tests.test_syntax.blocks.test_html_blocks.TestHTMLBlocks.test_raw_custom_element tests.test_syntax.extensions.test_md_in_html.TestMdInHTML.test_custom_element_markdown_attr tests.test_syntax.extensions.test_md_in_html.TestMdInHTML.test_custom_element_md1 tests.test_syntax.extensions.test_md_in_html.TestMdInHTML.test_custom_element_inline
• Full suite previously green under grok (python -m unittest discover tests)

Fixes #1246

[waylan]

If I am not mistaken, this is not the fix suggested in #1246. This accepts any tag name which contains a hyphen, whereas the fix discussed in #1246 was to allow the user to add specific tags and only those tags would be recognized as block-level. As noted, users can already add tags to the list of recognized tags. However, due to a bug, the md-in-html extension does not recognize them.

Note that, as documented, all first-party extensions have entered maintenance mode and are no longer accepting new features. Adding support for all hyphenated tags would be a new feature. However, fixing the bug which prevents the extension from recognizing a user provided tag (regardless of whether it was a web-component) would not be a new feature, but a bug fix (as that is how it was intended to work from the beginning). Therefore, that is the only fix we will accept. Anything more than that would require a new or forked third-part extension to implement.