feat(lua)!: execute Lua with "nvim -l"

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_.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/nvim/testdir/test_startup.vim b/src/nvim/testdir/test_startup.vim
index ee0445c6b4..54b7636f69 100644
--- a/src/nvim/testdir/test_startup.vim
+++ b/src/nvim/testdir/test_startup.vim
@@ -962,6 +962,7 @@ endfunc
 
 " Test for enabling the lisp mode on startup
 func Test_l_arg()
+  throw 'Skipped: Nvim -l arg differs from Vim'
   let after =<< trim [CODE]
     let s = 'lisp=' .. &lisp .. ', showmatch=' .. &showmatch
     call writefile([s], 'Xtestout')