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_.

src/nvim/globals.h

@@ -515,7 +515,7 @@ EXTERN int allbuf_lock INIT(= 0);
 /// not allowed then.
 EXTERN int sandbox INIT(= 0);
 
-/// Batch-mode: "-es" or "-Es" commandline argument was given.
+/// Batch-mode: "-es", "-Es", "-l" commandline argument was given.
 EXTERN int silent_mode INIT(= false);
 
 /// Start position of active Visual selection.