read: two hard breaks in a row become a paragraph break

[willkg]

Two hard breaks in a row read back as a paragraph break. Found while working on #204; reproduced on main:

storage	read writes	publish sends
<p>a<br /><br />b</p>	a␣␣\n␣␣\nb	<p>a</p><p>b</p>

The second break's own two spaces make a line that holds nothing but whitespace, and Markdown reads that as a blank line, so one paragraph comes back as two. The editor writes this shape for Shift+Enter pressed twice.

Since #204 the same happens when a <br /> at the end of a bold run is followed by a <br /> (<p><strong>a<br /></strong><br />b</p>), because the edge break now moves out of the mark instead of being dropped.

Notes

• A backslash hard break survives on a line of its own: a␣␣\n\\\nb publishes as <p>a<br />\n<br />\nb</p>. So the fix is probably to write every break after the first in a run as \ + newline.
• Breaks are compared and trimmed as the literal " \n" in several places (renderInlineRun, renderMark's edge trim, renderCellLines' <br> substitution, the heading's <br /> substitution), so each place needs to recognise the second spelling too. Otherwise a mark's edge trim stops at the backslash.