TSHighlighter now places marks for conceal_lines metadata. A new
internal decor provider callback _on_conceal_line was added that
instructs the highlighter to place conceal_lines marks whenever the
editor needs to know whether a line is concealed. The bundled markdown
queries use conceal_lines metadata to conceal code block fence lines.

Implement an extmark property that conceals lines vertically.

Add a new field `virt_lines_overflow` that enables horizontal scrolling
for virtual lines when set to "scroll".

Problem:
nvim_get_autocmds cannot filter by id.

Solution:
Support it.

This has been possible in the "backend" for a while but
API was missing.

Followup: we will need a `details2=true` mode for `nvim_get_hl_id_by_name`
to return information in a way forward compatible with even further
enhancements.

Problem:  We want to deprecate `nvim_err_write(ln)()` but there is no
          obvious replacement (from Lua). Meanwhile we already have
          `nvim_echo()` with an `opts` argument.
Solution: Add `err` argument to `nvim_echo()` that directly maps to
          `:echoerr`.

Problem: incorrect return type doc causes luals `Annotations specify that at most 0 return value(s) are required, found 1 returned here instead.` diagnosis

Solution: correct return type doc