and sync queries from nvim-treesitter (adds support for `MISSING` nodes).

Problem:
Headings in :help do not stand out visually.

Solution:
Define a non-standard `@markup.heading.1.delimiter` group and
special-case it in `highlight_group.c`.

FUTURE:
This is a cheap workaround until we have #25718 which will enable:
- fully driven by `vimdoc/highlights.scm` instead of using highlight
  tricks (`guibg=bg guifg=bg guisp=fg`)
- better support of "cterm" ('notermguicolors')

Problem: Neovim bundles treesitter parsers for bash and python but does
not use them by default. This dilutes the messaging about the bundled
parsers being required for functionality or reasonable out-of-the-box
experience. It also increases the risk of query incompatibilities for no
gain.

Solution: Stop bundling bash and python parser and queries.