Preserve bullet used for lists (-, * or +)

[max-nextcloud]

Describe the bug
There are different bullets chars that can be used to write lists in markdown. Preserve them when editing the file in text.

To Reproduce

1. Create a file containing a list using - chars as bullets.
2. Open the file in text.
3. Change and save the file.
4. Inspect the file. All bullets turned into *.

Expected behavior
Preserve the type of bullet used.

[max-nextcloud]

Text reads the markdown source and turns it into data structures for the tiptap editor.
When saving the file it serializes these data structures into markdown again.

In order to preserve properties of the initial markdown they need to be handled in each of the steps involved:

1. Parsing with markdown-it (md -> html)
2. Loading with tiptap (html -> tiptap nodes and marks)
3. Processing done by tiptap / prosemirror
4. Serializing with prosemirror-markdown (tiptap nodes -> md)

We already loose the information what bullet was used in 1.
You can try this on the demo site of markdown-it:
No matter which bullet you pick the html code always looks the same.
But the internal representation keeps track of the bullet in markdown:

debug view on demo site

[
  {
    "type": "bullet_list_open",
    "tag": "ul",
    "attrs": null,
    "map": [
      0,
      2
    ],
    "nesting": 1,
    "level": 0,
    "children": null,
    "content": "",
    "markup": "-", // <--- here.
    "info": "",
    "meta": null,
    "block": true,
    "hidden": false
  },
]

So what it would take would be turning this markup property of the token into an html attribute such as in <ul data-bullet='+'>.
We are already doing something similar for the underlined text.

In 2. we'd than have to convert this into an attr on the tiptap list node. Tiptap and prosemirror should leave this alone in 3. and 4. would be handled by prosemirror-markdown if the bullet was stored in the bullet attribute of the ul node.

[susnux]

Keep in mind that all list items should use the same bullet, e.g. new created items should use the bullet of the previous item. The same is valid for TODO lists.

[max-nextcloud]

@jancborchardt pointed out in the design review:

We can use style="list-style-type:'-';" with a string value in the HTML: https://developer.mozilla.org/en-US/docs/Web/CSS/list-style-type

[juliusknorr]

But in the rendering we actually want a unified look, right?

[max-nextcloud]

Frankly... i don't know. I'd be fine with both. Having different bullets might be confusing because there is no way of creating them (thus far) and to me it seems unnecessarily complex. Otoh if the rendered output closer matches the source might be nice.