[JammingBen]

Description

Stacked on #3616 and #3618.

• Leave numeric ordered lists to marked, so items indented less than the first one are no longer dropped. Letter and roman lists stay with tiptap.
• Start a list at an empty first item followed by whitespace (- , 1. ), which marked reads as paragraph text.
• Keep task syntax in ordered lists (1. [ ] task) as text.
• Make sure every list item starts with a paragraph, so empty sub items and their parents survive. A list item starting with another block, e.g. an ATX heading or a code block, gets an empty paragraph before it.
• Save empty list items as a bare marker (-, 1.) where possible, " " only where a bare marker would not parse back as an empty item.
• Keep inline formatting of text that follows a block inside a list item, and keep image-only list items.

Many of these workarounds can be removed once the referenced tiptap issues are fixed upstream (see linked issues in code).

Issue

fixes parts of #3583

Known limitations

• Setext headings (Title + ===/--- underline) as the first block of a list item are kept as paragraph text, not as a heading. Before this change they produced an invalid document. ATX headings (# Title) stay headings.
• An empty list item directly after a paragraph line (foo\n1. ) starts a list, while CommonMark treats it as paragraph text.
• Changing the delimiter within a numeric list (1) a + 2. b) splits it into two lists, as in CommonMark. Both are saved with ., so they merge back into one list on the next load (Text editor changes list structure of some markdown documents on save #3622).
• A single - below the first line of a list item (- Title + -) becomes an empty sub item instead of a setext heading underline. This keeps lists saved by earlier versions (- Parent + -) intact.
• Content under a numeric list item must be indented to the item's content column (3 spaces for 1. ), as in CommonMark. Content indented with 2 spaces, as saved by OpenCloud v7.x, is moved out of the list (Text editor changes list structure of some markdown documents on save #3622).