Add --list flag to to md for ordered/unordered/none list formatting (#17250)

fixes: #13762 

## Release notes summary - What our users need to know

**Breaking change:** `to md` now formats lists as unordered Markdown
lists (`* item`) by default.

  New `--list` flag controls list formatting:
  - `--list ordered` - numbered lists (`1. `, `2. `, ...)
  - `--list unordered` - bullet lists (`* `) (default)
  - `--list none` - plain text, no markers (previous behavior)

  Examples:
  
```nu
  [a b c] | to md                  # => "* a\n* b\n* c"
  [a b c] | to md --list ordered   # => "1. a\n2. b\n3. c"
  [a b c] | to md --list none      # => "a\nb\nc"
```

## Tasks after submitting
<!-- Remove any tasks which aren't relevant for your PR, or add your own
-->
- [ ] Update the
[documentation](https://github.com/nushell/nushell.github.io)

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Co-authored-by: Copilot <[email protected]>

Author: hustcer