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/lua/executor.c

@@ -323,6 +323,34 @@ static int nlua_thr_api_nvim__get_runtime(lua_State *lstate)
   return 1;
 }
 
+/// Copies all args into the Lua `arg` global.
+///
+/// Example:
+///     nvim -l foo.lua -- -e "sin=math.sin" script a b
+///
+/// @note `lua` CLI sets _negative_ `arg` indices to the arguments upto "-e".
+///
+/// @see https://www.lua.org/pil/1.4.html
+/// @see https://github.com/premake/premake-core/blob/1c1304637f4f5e50ba8c57aae8d1d80ec3b7aaf2/src/host/premake.c#L563-L594
+///
+/// @returns number of args (stops at "--")
+int nlua_set_argv(char **argv, int argc)
+{
+  lua_State *const L = global_lstate;
+  lua_newtable(L);
+  int i = 0;
+  for (; i < argc; i++) {
+    if (strequal("--", argv[i])) {
+      i--;
+      break;
+    }
+    lua_pushstring(L, argv[i]);
+    lua_rawseti(L, -2, i + 1);
+  }
+  lua_setglobal(L, "arg");
+  return i + 1;
+}
+
 static void nlua_schedule_event(void **argv)
 {
   LuaRef cb = (LuaRef)(ptrdiff_t)argv[0];