Add room.send_markdown API

It seems it's not possible to send markdown messages as of today.

Will you consider adding support for it, or is there any workaround (like converting markdown to html automatically then use `send_html`?

Start by locating the existing room.send_html API and reviewing how room message-sending entry points are tested. Check Matrix's markdown and HTML message-format expectations, including the workaround mentioned in the issue. Done means room.send_markdown is available with documented behavior and tests cover its message output.

Written by the indexing model from the issue text.