Cycle bullets when printing nested Markdown list

[tecosaur]

It's nice to visually differentiate different levels of nesting (and actually print more than one). Closes #18615.

With this markdown list as an example:

- some list
  - with nesting
  - and items
    - at many levels
  - some text
    - meaningless words
      - even more words
- yet another item
  - am I really still going?
- this is the end

It's currently printed like this:

    •  some list
       • with nesting
       • and items
       • at many levels
       • some text
       • meaningless words
       • even more words

    •  yet another item
       • am I really still going?

    •  this is the end

with this PR, it's printed like this:

    •  some list
       ➤  with nesting
       ➤  and items
          –  at many levels
       ➤  some text
          –  meaningless words
             ➤  even more words
    •  yet another item
       ➤  am I really still going?
    •  this is the end

This PR:

• fixes the 1-limited nesting bug
• fixes a bonus bug with non-loose nested lists getting extra newlines, and,
• introduces depth-dependent bullets

On my end, this seems to e working well, but it would be good if people could try this out and check I haven't accidentally introduced any edge-cases.

[KristofferC]

Maybe it is just my fonts but the second level arrows has a lot more "weight" to them and sort of overpowers all the other bullets:

[tecosaur]

Oh that's quite different to what I see:

Is it worth checking if that translates to what people might see in their terminals? If there's a risk of something like your screenshot, we'll definitely want to change the bullets.

[KristofferC]

(this was the github markdown rendering on an android phone FWIW)

[fingolfin]

Maybe imitate what GitHub uses?

Suggested change

	const _list_bullets = ["• ", "➤ ", "– "]
	const _list_bullets = ["• ", "◦ ", "▪︎ "]