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/main.h

@@ -23,15 +23,17 @@ typedef struct {
   char cmds_tofree[MAX_ARG_CMDS];       // commands that need free()
   int n_pre_commands;                   // no. of commands from --cmd
   char *pre_commands[MAX_ARG_CMDS];     // commands from --cmd argument
+  char *luaf;                           // Lua script filename from "-l"
 
   int edit_type;                        // type of editing to do
   char *tagname;                        // tag from -t argument
   char *use_ef;                         // 'errorfile' from -q argument
 
   bool input_isatty;                    // stdin is a terminal
+  bool input_istext;                    // stdin is text, not executable (-E/-Es)
   bool output_isatty;                   // stdout is a terminal
   bool err_isatty;                      // stderr is a terminal
-  bool input_neverscript;               // never treat stdin as script (-E/-Es)
+
   int no_swap_file;                     // "-n" argument used
   int use_debug_break_level;
   int window_count;                     // number of windows to use