markdown: Parse included blocks in a new parser state.
markdown: Parse included blocks in a new parser state.

This fixes inclusion of a multi-paragraph file into a list item.

Followup to commit dc33a0ae6734f47f407a0cdc15a94ef488da6e47 (#22315).

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip