bug(commonmark) Header html tags in table cells do not round-trip or preview properly

[yellis]

Describe the bug
Header html tags used in table header or body cells end up not previewing properly in RT mode and mangle the table upon round-trip. It works in current editor when saved to the site - renders properly.

To Reproduce

1. MD mode, create table, add an h1 html tag in any cell around the text
2. Go to RT mode, observe that the tag has been transformed as a literal
3. Go back to MD mode and the table syntax is all messed up and mangled

Expected behavior
Ideally we would support header elements in table cells. The GH MD table spec does not mention it, but the current editor and on-side rendering support it. If we decide not to support it, should at least allow round-trip without destroying the table markdup.

Screenshots

Desktop (please complete the following information):
Windows 11 Chrome 103

Additional context

• https://meta.stackexchange.com/a/380346/51
• https://meta.stackexchange.com/a/363974/51