Problem:
Nvim has Lua but the "nvim" CLI can't easily be used to execute Lua
scripts, especially scripts that take arguments or produce output.

Solution:
- support "nvim -l [args...]" for running scripts. closes #15749
- exit without +q
- remove lua2dox_filter
- remove Doxyfile. This wasn't used anyway, because the doxygen config
  is inlined in gen_vimdoc.py (`Doxyfile` variable).
- use "nvim -l" in docs-gen CI job

Examples:

    $ nvim -l scripts/lua2dox.lua --help
    Lua2DoX (0.2 20130128)
    ...

    $ echo "print(vim.inspect(_G.arg))" | nvim -l - --arg1 --arg2
    $ echo 'print(vim.inspect(vim.api.nvim_buf_get_text(1,0,0,-1,-1,{})))' | nvim +"put ='text'" -l -

TODO?
  -e executes Lua code
  -l loads a module
  -i enters REPL _after running the other arguments_.

This will check if the string after the variable in a @param is either
"number", "string", "table", "boolean" and "function" and if so add a
parenthesis around it. This will help separate the variable type with
the following text. Had all our functions been annotated with emmylua
then a more robust solution might have been preferable (such as always
assuming the third string is parameter type without making any checks).
I believe however this is a clear improvement over the current situation
and will suffice for now.

Co-authored-by: Elias Alves Moura <eliamoura.alves@gmail.com>
Co-authored-by: venkatesh <shariharanvenkatesh@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Vikas Raj <24727447+numToStr@users.noreply.github.com>
Co-authored-by: Steve Vermeulen <sfvermeulen@gmail.com>
Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Co-authored-by: rwxd <rwxd@pm.me>
Co-authored-by: casswedson <58050969+casswedson@users.noreply.github.com>