feat: Support section-end mark (line with only hashes)

[MurakamiShinyu]

This adds the following rule on sectionization:

• A line with only #s can be used to end the section whose depth matches the number of the #s.
  • e.g., the section starting with ### Heading 3 can end with ###.

(related to https://github.com/vivliostyle/vfm/issues/155, thanks @nosuke23 for the suggestion)