feat: support pandoc fancy_lists (I. / A. / a. / i. / (1)) behind an opt-in preference - #5754

With fancyLists on, a block tokenizer reads ordered lists that open with
an alpha or roman marker, or with a (1) style marker, following pandoc:
two spaces after a capital letter or initial-like numeral with a
period, later items read in the first item's style, a new list on any
change of style or delimiter, nested lists starting at one, and no
interruption of a paragraph. Decimal and bullet items also end at a
fancy marker. Ordered list state keeps the style, delimiter and start.

Ordered list items are written in their list's style and delimiter,
with two spaces after markers such as I. and A. that pandoc would
otherwise read as initials. The wider marker moves continuation and
nested lines to the new content column. Adjacent lists are told apart
by style as well as delimiter, and two ordered lists in a row with the
same markers are separated by an HTML comment so they read back as two
lists. The tests check each fixture against pandoc when it is
installed.

The markdown serializer takes a fancyLists option, passed from the
editor options. With it off, two adjacent ordered lists with the same
markers are written as before.

Ordered lists carry their style and delimiter as data attributes, and
the stylesheet maps them to counter styles, so a. IV) and (1) lists
show their own markers instead of decimal ones. A Chromium e2e test
reads the marker text from the accessibility tree.

Tab makes a sublist that numbers from one. Under an upper-roman list
it takes the outline marker for its depth (I., A., 1., a., i., (1),
(a)); other lists keep the parent marker. Outdent and split compare
the list style as well as the delimiter, so items of a different style
are not merged into the same list. Turning an ordered task list into
an ordered list from the front menu keeps its style.

With fancyLists on, a paragraph whose first line starts with a marker
such as I.  A.  a. i. (1) (a) or c) becomes an ordered list in that
style. Capital letters and initial-like numerals with a period need two
spaces, so B. Russell stays text, and a list inside a list item has to
start at one. Converting an item to a task item keeps the list style.

With fancyLists off, Tab copies the parent list's start number into
the new sublist as before.

With fancyLists on, HTML export, static rendering and clipboard HTML read
pandoc fancy lists and write each ordered list as <ol type> plus a class
naming its style and delimiter. Export CSS draws the ) and (1) markers with
@counter-style and keeps nested typed lists from being forced to decimal.

The markdown reader used for backslash math switched off every extension
the gfm listing does not name. gfm reads fenced code, intraword
underscores, backslash line breaks, shortcut reference links and start
numbers without listing them, so those exports turned code blocks into
inline code and lost list start numbers.

Preferences > Markdown gets a switch for pandoc's fancy_lists extension,
off by default. Toggling it re-parses the open document so lettered and
Roman numeral lists appear or turn back into text.

With fancyLists on, Convert with Pandoc reads the document with pandoc's
markdown reader, shaped like gfm, plus fancy_lists. pandoc's gfm reader
takes +fancy_lists but folds an indented A. item into its parent's text,
so an outline only nests when markdown reads it.

With fancyLists on, the / quick-insert menu and Paragraph > Outline
(Cmd+Option+K on macOS, Ctrl+Alt+K on Linux) insert an ordered list
in upper-roman style, turn the list at the cursor into one, or wrap a
multi-block selection into one. Windows gets no default shortcut because
Ctrl+Alt types characters there. The entry is hidden while the option is off.

Copied markdown now ends a list with <!-- --> before another list in the
same style, as saving does, so pasting it reads back two lists.

The block menu builds its Turn Into list from the quick-insert entries
through canTurnInto, so a paragraph already offers Outline with its icon
while fancyLists is on. Pin that, and that choosing it writes I.  text.

Launch the app with fancyLists on and check that an outline renders as
nested typed lists, that Tab and Shift+Tab move an item along the outline
ladder, that Enter continues roman numbering, and that the markers stay
paragraph text with the preference off.

Add the fancyLists preference to the preferences reference, and a Fancy
lists section to the syntax guide covering the markers, the two-space rule,
the outline levels Tab moves through, and the <!-- --> list separator.

With fancyLists on, #., #) and (#) open an ordered list in pandoc's
default style. The editor and HTML export number it in digits, and a
save keeps the # markers, also after an edit. Typing #. and a space
starts one. As in pandoc, a # item continues a list of any style, a #
list continues with digits, and adjacent lists that would read back as
one are ended with <!-- -->.