getMarkdownHeadersInCell does not handle headers with HTML tags

[DonJayamanne]

The notebookOutlineEntryFactory.ts handles markdown headers with HTML tags, but not other parts (such as collapsing mardown header cells, see foldingModel.ts).

			const fullContent = cell.getText().substring(0, 10000);
			for (const { depth, text } of getMarkdownHeadersInCell(fullContent)) {
				hasHeader = true;
				entries.push(new OutlineEntry(index++, depth, cell, text, false, false));
			}

			if (!hasHeader) {
				// no markdown syntax headers, try to find html tags
				const match = fullContent.match(/<h([1-6]).*>(.*)<\/h\1>/i);
				if (match) {
					hasHeader = true;
					const level = parseInt(match[1]);
					const text = match[2].trim();
					entries.push(new OutlineEntry(index++, level, cell, text, false, false));
				}
			}

Thus, if we have HTML tags for outline headers, then we do not render collapsible regions in markdown for such headers.
Not sure if this is a bug or not, but filing this here,

Feel free to close this if this is invalid.

[Yoyokrazy]

Good catch, I'll see about unifying that behavior. We should likely just support html tags in the folding model. Might've just been an oversight way back when this was written and didn't grab the updates with the outlineEntryFactory.