Add support for line blocks

[Witiko]

jgm#41 implements Pandoc's line blocks. Perhaps we can rebase their implementation on witiko/markdown?

[alperozoner]

Adding support for witikio won't help; it should be backported to pre-jgm#41.

[Witiko]

@alperozoner: If you are saying that we should merge jgm#41 to jgm/lunamark first, then I have that planned for Wednesday after I have reviewed it. Then, I will rebase the changes from jgm#41 on witiko/markdown.

[Witiko]

Assigned to @lostenderman as part of grant project MUNI/33/1658/2022.
Tentatively scheduled for milestone 2.20.0 to be delivered by January 31, 2023.

[Witiko]

@lostenderman For an overview of what goes into creating a built-in syntax extension, see:

• Add superscript and subscript syntax extensions #162
• Add fancy_lists syntax extension #168
• Add support for div and span attributes #207

In general, there are 5 steps:

1. Implement built-in syntax extension in Lua (e.g. M.extensions.fancy_lists).
2. Add and document an option that enables the syntax extension (e.g. fancyLists).
3. Add and document renderers that will be used to display new elements (if any) in TeX (e.g. \markdownRendererFancyOlItem).
4. Unit-test the syntax extension:
   1. Add new renderers (if any) to unit tests, see tests/support/plain-setup.tex and the witiko/markdown/test LaTeX theme.
   2. Add unit tests (e.g. fancy-lists.test and no-fancy-lists.test).
5. Update CHANGES.md.

Optionally, we also:

6. Define how the renderers should look in plain TeX, LaTeX, and ConTeXt by default.
7. Add the new syntax extension to example documents for LaTeX and ConTeXt.