We must not remove link reference definitions until we check for list
tightness.  This commit defers resolving of link reference definitions
until finalization of the document.  We still need to eagerly remove
link reference definitions in setext headings to determine whether it is
a setext heading or a thematic break.  So this commit provides slightly
different functions for resolving link reference definitions and
checking if a line is blank for `cmark_strbuf` and `cmark_chunk`.

This flag was introduced by
commonmark#284, but we will not need it
once we update `S_ends_with_blank_line` to not use resursion in the next
commit.

- Set the end position precisely
- Check list tightness by comparing line numbers
- Remove `LAST_LINE_BLANK` flag

See also commonmark/commonmark.js#269 .

Classification of end positions:

- The end of the current line:
  - Thematic breaks
  - ATX headings
  - Setext headings
  - Fenced code blocks closed explicitly
  - HTML blocks (`pre`, comments, and others)

- The end of the previous line:
  - Fenced code blocks closed by the end of the parent or EOF
  - HTML blocks (`div` and others)
  - HTML blocks closed by the end of the parent or EOF
  - Paragraphs
  - Block quotes
  - Empty list items

- The end position of the last child:
  - Non-empty list items
  - Lists

- The end position of the last non-blank line:
  - Indented code blocks

The first two cases are handed by `finalize` and `closed_explicitly` flag.

Non empty list items and lists are handled in `switch` statements in `finalize`.

Indented code blocks are handled by setting the end position every time
non-blank line is added to the block.