feat: add extmarks for markdown list markers

[sarmong]

#46

[bew]

mention that it replaces all kinds of list items?

[sarmong]

What do you mean by all kinds of list items? All types of list item markers?

IMHO, list markers already mean all.

[lukas-reineke]

I'm okay with adding this, but it should work for all filetypes, not just Markdown.

[sarmong]

@lukas-reineke I don't know what we should do about neorg and org.

Neorg's unordered list items look like this and I don't think there is value in substituting them all.

    - Unordered list level 1
    -- Unordered list level 2
    --- Unordered list level 3
    ---- Unordered list level 4
    ----- Unordered list level 5
    ------ Unordered list level 6

Org treesitter doesn't differentiate between ordered and unordered lists, so it will not be possible.

[sarmong]

I found a bug that when you horizontally scroll the window and the first column is invisible, the extmark is still shown on the first visible column. Will look into it when I have time.

[cultab]

Neorg's unordered list items look like this and I don't think there is value in substituting them all.

iirc Neorg has it's own conceal module included that substitutes them.