Before the 7121983c45d92349a6532f32dcde9f425e30781e refactoring this
loop added highlights from a `buf_hls` list that had filtered out
elements with `Attrs.None`. After the refactoring this added highlights
from `hls` directly, and those elements would fail with e.g.:

    $ nvim 'man://math.h(0)'
    Error detected while processing command line:
    Error executing Lua callback: /usr/share/nvim/runtime/lua/man.lua:205: Invalid 'hl_group': Expected Lua string
    stack traceback:
            [C]: in function 'nvim_buf_add_highlight'
            /usr/share/nvim/runtime/lua/man.lua:205: in function 'highlight_man_page'
            /usr/share/nvim/runtime/lua/man.lua:632: in function 'init_pager'
            /usr/share/nvim/runtime/plugin/man.lua:9: in function </usr/share/nvim/runtime/plugin/man.lua:6>

- Replace all uses of vim.regex with simpler Lua patterns.
- Replace all uses of vim.fn.substitute with string.gsub.
- Rework error handling so expected errors are passed back via a return.
  - These get routed up an passed to `vim.notify()`
  - Any other errors will cause a stack trace.
- Reworked the module initialization of `localfile_arg`
- Updated all type annotations.
- Refactored CLI completion by introduction a parse_cmdline()
  function.
- Simplified `show_toc()`
- Refactor highlighting
- Inline some functions
- Fix completion on MacOS 13 and earlier.
  - Prefer `manpath -q` over `man -w`
- Make completion more efficient by avoiding vim.fn.sort and vim.fn.uniq
  - Reimplement using a single loop

Problem:
`man -w` does not work on recent versions of MacOs.

Solution:
Make it so an empty result is interpreted as an error unless silent=true

This reverts commit 7940ec69136fa992c98aa7b37265fbc2e619232e.

closes: #31512

* deprecate old signatures
* move to new str_byteindex/str_utfindex signature
* use single-underscore name (double-underscore is reserved for Lua itself)