Markdown: Enforce consistent bullet character for unordered lists

[MHLut]

Markdown supports multiple characters to denote unordered list items, notably a dash (-), asterisk (*), or a plus sign (+).

Proposal:

Add a linter rule that enforces using one of these three characters throughout Markdown files, and a formatter rule with an automatic fix.

Personal note:

I created this proposal after Biome corrected asterisks to dashes in a README. I do not use dashes for accessibility reasons, per: Quick tip: Formatting unordered lists in plain text.

[Netail]

To add to that, something I noticed with another formatter. Is a different character per list level, so you can easily differentiate which entry is part of which level

[nhedger]

Will be implemented as part of #12857