Fix list parser

[rafalp]

Markdown spec: https://spec.commonmark.org/0.31.2/#list-items

Misago's parser is quite behind the spec on this one:

• Doesn't support multiline list items
• Doesn't differentiate between tight and loose lists
• Doesn't support nesting blocks in list items
• Doesn't support custom start attribute on ordered lists

Also new parser problems introduced in #1882:

• non-indented next line should only be included in a list item if its not a block item