feat(lua): send "--" literally to Lua "-l" script

Problem:
When "-l" is followed by "--", we stop sending args to the Lua script
and treat "--" in the usual way. This was for flexibility but didn't
have a strong use-case, and has these problems:
- prevents Lua "-l" scripts from handling "--" in their own way.
- complicates the startup logic (must call nlua_init before command_line_scan)

Solution:
Don't treat "--" specially if it follows "-l".

src/nvim/lua/executor.c

@@ -323,32 +323,28 @@ static int nlua_thr_api_nvim__get_runtime(lua_State *lstate)
   return 1;
 }
 
-/// Copies all args into the Lua `arg` global.
+/// Copies args starting at `lua_arg0` into the Lua `arg` global.
 ///
 /// Example:
-///     nvim -l foo.lua -- -e "sin=math.sin" script a b
+///     nvim -l foo.lua --arg1 --arg2
 ///
 /// @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)
+/// @returns number of args
+int nlua_set_argv(char **argv, int argc, int lua_arg0)
 {
   lua_State *const L = global_lstate;
-  lua_newtable(L);
+  lua_newtable(L);  // _G.arg
   int i = 0;
-  for (; i < argc; i++) {
-    if (strequal("--", argv[i])) {
-      i--;
-      break;
-    }
-    lua_pushstring(L, argv[i]);
-    lua_rawseti(L, -2, i + 1);
+  for (; lua_arg0 >= 0 && i + lua_arg0 < argc; i++) {
+    lua_pushstring(L, argv[i + lua_arg0]);
+    lua_rawseti(L, -2, i + 1);  // _G.arg[i+1] = "arg1"
   }
   lua_setglobal(L, "arg");
-  return i + 1;
+  return i;
 }
 
 static void nlua_schedule_event(void **argv)