Markdown encoder/decoder doesn't recognize "-" as start of unordered list item

[adriank]

Steps to Reproduce

final doc = const ParchmentMarkdownCodec().decode('''
paragraph
- a
- b
- c
''');

The unordered list is not recognized.

line 39 is:

static final _ulRegExp = RegExp(r'^( *)\* +(.*)');

and should be:

static final _ulRegExp = RegExp(r'^( *)[-\*\+] +(.*)');

There is also

    ParchmentAttribute.ul: '* ',

which should probably be customizable, since it depends heavily on locale and context (for example, in books - is commonly used, not *).

Here is markdown recommendation: https://www.markdownguide.org/basic-syntax/#unordered-lists. Unordered list items can start with -, *, and +.