Work towards enabling ["flow" layout](https://github.com/neovim/neovim/blob/9244da12e400ec0291450a5e9400d995e7a29b83/src/gen/gen_help_html.lua#L86-L88)
in all `:help` files.

Problem:
The final boss of vimdoc pseudo-structure: Vim's quasi-headings on
function signatures and excmd docs is visually distinctive but too
inconsistent for a nice parse rule.

Solution:
- Define the quasi-heading format as an "h4" variant, specified at
  `:help help-tag-heading`.
  - For simplicity and predictability, it is defined as any line that
    ends with a help tag(s) preceded by 2 or more spaces (or 1 tab).
- Update existing docs.
  - This one-time update has a lot of changes, but is very unlikely to
    conflict with ported Vim-patches because (1) Vim rarely adds new
    *excmds* that we want, and (2) *function* docs are auto-generated.

ref https://github.com/neovim/tree-sitter-vimdoc/issues/110

Work towards enabling "flow" layout in most `:help` files.

Since [tables are supported in help](https://github.com/neovim/tree-sitter-vimdoc#tables),
it's now possible to render most help files as ["flow" layout](https://github.com/neovim/neovim/blob/9244da12e400ec0291450a5e9400d995e7a29b83/src/gen/gen_help_html.lua#L86-L88)
instead of the legacy fixed-width layout. Besides [improved HTML rendering](https://github.com/neovim/neovim/pull/42381#discussion_r4285705739),
flow layout will allow `:help` presentation-mode to un-hardwrap text
blocks.

highlights.scm: `#match` is slow, prefer `#lua-match`.

Generate PDF using `chrome`.
TODO: we probably don't need the `typst` codepath.

Render tables in both `:help` and generated HTML.

Use NBSP (nonbreaking space) char in empty table cells.

Problem:
- `<` before a listitem (`< - foo`) is not concealed.
- Codeblock HTML has extra blank lines.
- Codeblock in a listitem ends the listitem, so HTML docs drop the rest
  of the item and its nesting. Example from `:h nvim_open_win()`:
  ```
        • border: … the double-box style can be specified as: >
          [ "╔", "═" ,"╗", "║", "╝", "═", "╚", "║" ].
    <
          If fewer than eight chars are given, they will be repeated.
        • bufpos: …

    tree-sitter-vimdoc v4.1.0:
      (block
        (line_li (prefix) (line) (codeblock (code (line)))))
      (block
        (line)                                  ; "If fewer…"
        (line_li (prefix) (line) (line)))       ; "bufpos"

    tree-sitter-vimdoc v4.2.0:
      (block
        (line_li (prefix) (line) (codeblock (code (line)))
          (line))                               ; "If fewer…"
        (line_li (prefix) (line) (line)))       ; "bufpos"
  ```

Solution:
- Bump tree-sitter-vimdoc to v4.2.0. https://github.com/neovim/tree-sitter-vimdoc/pull/174
- gen_help_html: get listitem indent from its first line
- gen_help_html: don't double codeblock newlines.
- highlight query: conceal `<` in `line_li`.
- docs: fix doc sources where a blank line was missing after listitems
  followed by non-listitem text.

Problem:
gen_help_html.lua only outputs HTML, but Typst is useful for producing PDFs.

Solution:
Add gen_one_typ() and ts_node_to_typ(), which mirror the existing
gen_one_html() and ts_node_to_html().

Problem:
The list-item indentation logic in `gen_help_html.lua` tracks `opt.indent` that
only decrements by one level when a list item is less indented than its sibling.
Dedenting across multiple nesting levels at once produces the wrong left margin.

Solution:
Track the exact leading whitespace for each indent level in a `opt.indent_ws` stack:
- On reset or a top-level item initialize the stack with the current item's
  whitespace.
- When indenting deeper push the current whitespace at the new level.
- When dedenting pop every level whose tracked whitespace is deeper than the
  current item (reaching the correct ancestor level).

Problem:
We'd like to export a PDF version of the user manual
(https://github.com/neovim/neovim.github.io/issues/477), but
`gen_help_html.lua` assumes an HTML output in several places.

Solution:
- Make gen() take an output param
- Rename gen_one() -> gen_one_html()
- Rename visit_node() -> ts_node_to_html()
- Add a mapping to gen() that calls the correct gen_one*() function.

This prepares the addition of new methods for Typst output.